this might be a little late, but the correct way to do this (with any tub) is to assure that both long sides of the tub (the sides with the green arrows pointing out when you click on the home store/move option) are not against a wall or any other piece of furniture/plant. basically, even though the adult sim will sit on the same side as the toddler enters the tub, the game needs space for 2 sims to use the same tub (hence why you have to have both sides accessible).