I actually love this idea for an AU. It's a nice change of pace from the dozens of invasive Yuri thoughts (though I enjoy that too). I can imagine it as Cadance having to be tested just like Twilight, but in her case, Cadance has to raise a Changeling before Changelings were ever known to be capable of change. Not only that, but the future queen of Changelings!
Princess Cadance and Shining Armor might certainly have their doubts, but if Celestia thinks it's possible, they can't just say no. Besides, they were hoping to have a child some day anyways and this would be good practice, right? If they can successfully raise a Changeling, then surely they can raise a normal pony! (Hah! A normal pony? Oh those sweet summer horses)
Anyhoodle, Chrysalis would start off as a huge pain and would definitely continue to be a pain here and there, but she'd have her moments. Perhaps something of an instinctual brat that can't control her hunger at first and Shining and Cadance have to work on reining it in somehow. She's younger than Thorax, so it would probably take her longer to get to the point of being able to no longer feel starved without draining love.
I can imagine a few episode ideas following her childhood and the struggles of raising her. First and foremost is the family getting used to each other, complete with Chrysalis giving into instinct and trying to drain someone, which immediately gets her a scolding, maybe another punishment of some kind. Possibly make the lesson about finding ways to truly teach a child right from wrong and about consequences, because sometimes parents don't realize that punishment without understanding can lead to worse things.
There can be an episode about bullying, as I can imagine Chrysalis wouldn't be the sort to be nice to others her age at first. We can establish a friend for her, either it is someone she apologizes to for picking on, or she can be a bully for a while, gain a rival and later friend as a result of her faults early on. Another part of this idea would be her slowly coming to learn ways to get what she wants out of adults, like children tend to do once they start learning at an early age.
A big episode for Kid Chrysalis could involve her feeling upset and uncomfortable from starvation because she hasn't been allowed to feed off anyone. By now, Cadance could have become attached to her and want to help her, so offers a bit of her own love to help ease the pain, not wanting Chrysalis to ruin her own progress by feeding off others. This can culminate into Chrysalis slowly driving Cadance into a sluggish wreck over the course of the episode. Heck, this could even be a thing with her development in general, since knowledge about Changelings wasn't all that great in the beginning, so for all they know, Changelings could genuinely need to feed like that. Either way, the feeding starts to take its toll on Princess Cadance, and Chrysalis notices it more than anyone else. At first she denies it, scared of starving herself all over again as well as reasoning that "the Princess of Love should have tons of love, right? Like, infinity plus one!"
Maybe Cadance becomes bedridden and needs rest, leaving Shining Armor to dote on her and nurse her back to health for a while, whilst Cadance's duties get handed off to someone else for a bit, one of the other princesses most likely. When Shining Armor leaves the room to fetch her some soup, Chrysalis sneaks in to see her and seeing Cadance buried under blankets and barely able to move finally breaks it all down for her. Cadance spots Chrysalis and greets her sweetly, asking if she's hungry, but Chrysalis finally says no. She doesn't want to feed off of her anymore, if this is what's going to happen to her. Before, Chrysalis probably wouldn't have referred to Cadance and Shining Armor as her parents, likely having come up with nicknames or something else, maybe even just call them by name at first. This would be her moment to finally see Cadance as and refer to her as her mom, and it could bring them closer.
This wouldn't lead to her metamorphosis, mind you, but it would lead Chrysalis to start feeling less and less hungry over time. She wouldn't be too close to Shining Armor, I can imagine there being some tension between them that could stall their dynamic a bit. We can have some fun with Chrysalis's unique body horror-esque abilities here and there as we go, scaring ponies on Nightmare Night, being a little gross every now and again, etc. I can imagine she'd be in a tense part of her life as she grows older where she knows and appreciates what it feels like to be loved, but not know how to, and possibly even be scared of giving love back.
Also, we'd totally have an episode where Chrysalis gets jealous of Flurry Heart getting all of Cadance and Shining Heart's attention after she's born. I can imagine those two having an interesting dynamic, especially with Big Sis Ladybug having to help take care of Flurry. Oof, imagine Twilight having to babysit the both of them during that one episode! XD
Chrysalis getting a metamorphosis is an inevitability in this sort of AU, because if she's being raised to know love and friendship and knowing right from wrong, it's a given she'd eventually change, unless we're still going for that tragic subversion, but then where's the fun in that?