I would also disfavor scheming, cunning, and conniving applied to 99% of 7-year-olds. In my case it’s not because I dislike the words, particularly. The thing is, I’m sure there are some mature seven-year-olds who are hardened and cynical, but most seven-year-olds of my acquaintance aren’t, even the whiny and obnoxious ones who have learned terrible habits from their parents. And what I don’t like about it is that all four of those words (those three plus “manipulative”) imply that the kid is doing these actions consciously and cynically, and I just don’t think that’s true of most kids that age.
“Devious” I would mind if applied to whining to get one’s way (again because I don’t think 7-year-olds generally whine as part of a conscious nefarious plan; I would characterize it more as a habit formed because of parental reinforcement), but I wouldn’t mind it being applied to a kid in other ways, like, if my 8-year-old deviously got me to walk right into the punchline of a joke. Actually, this rarely happens, but I think it illustrates my point that if I said that, you’d assume that my kid consciously meant to get me to do that.
I suppose if you said “unconsciously manipulative” I would like that better. Though of course there may be some small subset of 7-year-olds who are doing it consciously, and that doesn’t capture it. I don’t know if there’s a word that means “manipulative, but probably doing it unconsciously.” Maybe “whiny” or “spoiled,” as @Beckdawrek says.