They’re sentient if the child thinks of them as such. Andy’s etch-a-sketch was used in play as a character, so it gets thoughts. The Lilypad, with its decorative face and hands, gets the same thing.

Really, the only gap in this logic I can think of in the toy story universe is the shipping crate of buzzes that no child has even seen yet, but besides them, toys tend to wake up and be alive when interacted with, either by a child or a toy that has itself come alive from a child. S Even sitting on a store shelf, a child will still see the toy and imagine on occasion, so this still works for Stinky Pete.