Okay, this is a tricky one. I got 'A Twisted Tale' for my niece, thinking it was just Disney with a twist, but some of them get surprisingly dark. 'As Old as Time' and 'Unbirthday' tackle some heavy family drama and societal themes.
I'd say the older middle-grade readers, maybe 12 and up, could handle most of them, especially if they're already into darker YA. The younger ones might be better off sticking to the core Disney stories. It really depends on the individual kid and which specific book you're looking at—'Part of Your World' is way lighter than 'What Once Was Mine', for instance.