I'm not sure a babysitter is the right example, as it is often an issue of conflicting values, rather than the babysitter actually being right, given the child's values. If you lie to a human-level paperclipper, it's not because you think they won't understand, but because you know they won't care.

A child can be fully aware that a babysitter has homework to do and still want to see the toys.

I wouldn't consider that lie justified anyway. If you're circumventing a child's preferences, you should tell them you're doing it, and why.