I doubt the issue with toddlers and staircases has anything to do with the way a house is built. I've had this happen to me in every possible house, including a huge mansion I built for a Dynasty. There was definitely enough room there for mom to pick up the toddler without any trouble, but no! she had to go downstairs. Then I would make the toddler move away from the top of the stairs, tell mom to pick him up again, and it would work... proving that they had enough room to do it. And it doesn't only happen when I try to teach them something or read them a book... also when I try to feed them, change them, put them to bed etc. One more reason to hate sim-toddlers, LOL. Lately, unless I'm playing a challenge, I just adopt children or use testingcheats to manage the baby/toddler's needs until they age up to children. So they basically stay in their room playing 24 hours a day. I know, I'm a cheater... but I hate sim babies and toddlers!!!!
Another thing I really hate are EA built houses. They are so ugly! And most of the times they are really unpractical, like wasting too much room for useless stuff.