Your child is standing on an empty carton box.
Because the box is not strong enough to support the child's body so his leg might get in the box.
Is it natural to say "don't step on that empty carton box as you might get your leg caught/trapped in it"?
I think "caught" or "trapped" are way too much for a carton box.
In other situation, "caught" or "trapped" are suitable, for example, “I accidentally got my leg stuck/caught/trapped in the drain grate.”