The heights thing actually makes a lot of sense, especially for babies. Number one, Sans probably isn't used to being held by anyone except Gaster, and Asgore is even taller, so the extra height is probably scary, especially if Gaster doesn't make a regular habit of lifting the kid in the air. And if Sans has an ocular problem at a young age, his perception from up in the air might make being up high a scary thing due to a distorted/warped sight. Kid might need glasses. Same thing happened with one of my own family members. Hated being lifted up in the air as a baby until they got glasses, and could actually see where they were.



But in Sans' case, it also might be an instinctive fear, as he only has one HP, and falling from a great height could potentially be fatal...