I've been having this problem for ages and it's been driving me nuts. If I have a 2+ story house and the child is upstairs and I try to feed him/her or teach it something (walking, talking etc.) then sometimes the adult sim will go downstairs, the toddler will sit at the top of the stairs and that's pretty much it My adult sims won't go back upstairs and are just standing there.

The only way I can fix it long-term is make sure the toddler has a crib downstairs but that's annoying/stupid.

I have the same exact problem. It's so annoying! Even when my sim actually gets the toddler downstairs with him/her, they still usually just stand there and stare at it instead of taking care of the problem. Or they feed the kid, then leave it downstairs all night to scream while they wander aimlessly around the second story.
The same thing happens for me when my sim wants to tutor another sim. Even when there is plenty of space, they insist upon going downstairs to sit on the floor.