Mdthbs

So, I have a couple moments in a fantasy story where I want the heroes' actions to actually have consequences (Well, that are really impactful to the story.) One is where the MCs kill a minor lord, causing a major disruption as the daughter takes over as a child ruler. Luckily, she's mature for her age, and will try to rule fairly and kindly, unlike her father, but that's another tale.

The other moment is when an MC, a prince thought to be dead, shows up in his
home country, and finds it in ruins. Well, a magician has gone on a year or two long power trip, and hadn't thought to actually rule the country, and therefore, all the petty nobles are controlling their own territories. I want there still to be hope in the story after these two events happen, but I also want there to be realistic consequences for their actions.

My question is: how do I portray realism without making the whole situation a s***show?

Realism means variety, because real life isn't all one thing

To some degree, you've answered your own question:

I want there still to be hope in the story after these two events happen

If a little kid's parents die, show him sometimes forgetting to mourn and having fun instead. If petty nobles end up ruling their fiefs unsupervised, show some of them actually caring for the commoners under them, and getting along okay. (Even if they're reasonably worried that they'll be helpless if the big bad neighboring noble notices they're not united, and might come snapping up their bit territories one at a time.)

If you want to avoid grimdark, just remember that realistic life, even in terrible circumstances, has variety, and some of that variety is pleasant. And show that things are not entirely hopeless. In particular, even when the larger arc is looking quite negative, small joys and small triumphs in the present go a long way to lightening the tone. ("My prince - you're really alive? Can it be true?")

That's more or less all there is to it.

They say you shouldn't show gore, if you want it to have an emotional impact. Instead, show a teddy bear, or some other child's toy, sitting abandoned, or placed by a grave.

The same can hold true for the opposite. You want to show it isn't grimdark, then show hope, show life. Kids playing in the streets as their mothers call them in for supper. Show women hanging their laundry in their back yards. Show a festival, with happy faces and music and dancing.

If the fiefs are now 'independent', show one of them thriving. Show a market place with fresh foods and goods. Show adventurers' guilds thriving (if that's part of the world). Show trade continuing.

But also show that some of the fiefs don't get along--disparaging comments about 'those' people kicking up trouble. Show petty squabbles between neighbouring towns, even.

It has to make sense in the bigger picture. So, don't show that life is just dandy, but don't let it be misery porn either. There needs to be something worth protecting, but there needs to be something that needs to change as well--that's where the story comes from, after all.

Realistically, life sucks, but most of us manage to find hope somewhere. Sometimes little girls have to grow up too fast and sometimes we have to deal with messes other people make.

It sounds like your young lady is going to have a lot of tears and fight some battles she's not ready for. Let her fight. Let her cry. But, make it worth it in the end. “That which does not kill us makes us stronger.” Don't forget to let her laugh sometimes too. Give her some battles she can win, so she gains the confidence to keep trying after the ones she loses.

Your long-lost prince is going to have to pull up his big boy pants and learn some diplomacy and negotiation skills. The entire structure of his kingdom has been altered and he's going to have to work with that. He may never have the same amount of control as the previous king, but that’s life.

Give both of your characters failures and successes. Heavy consequences for bad decisions aren't necessarily grimdark. They are learning experiences. They develop character and make the few victories that much sweeter.

As you plan out these successes and failures, some avenues of hope will emerge and, hopefully, you will begin to see the pathways towards a happy ending. Or, at least, an ending everyone can deal with.

Heroes Have Consequences.

Heroes cause major changes, and every major change is likely to be negative for somebody, and often that person is an innocent. No matter what the setting, defeating evil is meaningless if the evil is not ruining lives (or about to ruin them). It may take a war to defeat the evil, but in the process soldiers die on both sides, and the soldiers are the sons and daughters, fathers and mothers, brothers and sisters of innocents that were not evil, but caught up in forces far greater than themselves. When the righteous prevail in a war, they make the world a better place, but there is always a cost.

Being heroes, I presume they had no real choice but to kill the minor lord. He was ruining lives somehow, sacrificing the lives of his soldiers over petty personal disputes, taxing his people into poverty, or hanging them or crucifying them for dissent, or raping their daughters, whatever.

But in the process of making the world a better place for his subjects, they harmed an innocent -- They orphaned an innocent girl.

What you can do about it, as an author, is find a way to let them be heroes again. They don't have to kill the lord and walk away. Let's say, upon the lord's death, some member of his evil entourage has designs on taking his place -- He wants to murder the daughter to clear his path. And even though she blames them for his death, in front of her they save her life. She knows it. This proves to her which members of the court are loyal to the crown. Now the dilemma is on her, does she order the traitors killed? Orphan their wives and children? She does, because their crime must have public consequences, but at their execution she announces that their wives and children are henceforth wards of the crown and under her protection.

Your returning prince feels like he abandoned his people. Maybe he did, maybe he was thoughtless. He was off making the world a better place, but there are always consequences to innocents, including his people. He can't undo those consequences, but he is a hero, his friends are heroes. How can he, or the group, be heroes again in these circumstances? Can they kill the worst of the overlords? Is there some way to put this country back on the right track? Can they kill the idiot magician and install a diplomat that can reunite the country? Say, the one noble that has done the best job of running his territory for the good of his people, instead of himself. Even if the prince will leave again, perhaps his little band of heroes can forge an alliance of nobles by eliminating an obstacle in their way; a tyrant or two that separate them.

The heroes are your engine of hope, their job is to make the world better, even if they are the ones that screwed it up.

It doesn't mean they erase the consequences. The little girl is an orphan. Lives were ruined and lost in the country. But they step up and create a hopeful path for going forward.