I had this issue just today. Go into build mode. Delete the stairs. The toddler will relocate to the floor because the object has been deleted. Go to live mode and move the toddler out of the way. Re-place the stairs.
It's a pain but this is the way around it until it gets fixed.
Definitely have noticed this as well in my own 100 baby challenge. The house my family lives in has a staircase that turns 180 degrees in the middle, and occasionally a newly aged toddler will go up the bottom part of the staircase looking for an interaction with a family member that is upstairs and then gets stuck in the middle because the game remembers that the toddler is not supposed to be able to climb stairs yet.
