ooooh this is a tough question. When I was little I watched the Little Mermaid so much it broke (back in the days of VHS) so if you asked toddler me it would be that. Ask me now though and my favorite kids movie would probably be Mulan or Finding Nemo.
Ooohhh, I'd probably pick Nightmare Before Christmas too actually. But if I had to pick a second movie that I was obsessed with as a kid, I'd probably pick Robin Hood: Men In Tights. Though it was definitely NOT a children's movie lol.