Have always really liked David - and David Henesy was an amazing actor, IMO. Esp. liked him when he was posessed by Count Petofi in the 1897 storyline. Another fave is Amy Jennings - she was entirely believable, and fairly natural, considering.
I hate Hallie Stokes with white-hot passion, but not sure I really see her as a child. Hell, if she'd been only a couple years older, the powers-that-used-to-be might've even hooked her up with Barnabas!
So, for least favorite child on DS, I'm going to have to go with Michael-the-Leviathan. He was a right little sh*t!