I love the older versions of them, and Arjen and Arien nuzzling each other, really adorable. All they really got is the other (from this world and the next) and I love how they want to see the world when they are older because they never got a chance to really do so before now. I love how they are trying to get along and to reach out but misunderstandings do tend to happen.
They are ignorant to what's around them and who can blame them their bitch of a mother and abused them for five years. They are recovering but it's a lengthy process. Trust varies from person to person and it can be hard to tell sometimes. Especially when the King inadvertently played a hand in it, no doubt they would still be salty on that. They remember that.
It's a new world, one they are still learning about because again kind of difficult to know about the world when you were stuck in an abusive home for five years and the fact they died quite young in the past. If they were actual five-year-olds, I honestly don't think they will be talking or even be emotionally present and be even more distant with the King and the Princes. If not outright hostile.
Although abuse does tend to regress people's developments no matter how old they are.
They like Riolu because they sense a shared connection with him. They know probably what he's been through and they can relate with their own. Birds of a feather and all that jazz.
They just want to live and I can't wait to see it.