The walkways & platforms on the tube aren't level....that's what a brake on buggys are for.
Interestingly, the Metro this morning has the story that they had two buggys & the guy went to help the mother with the second while leaving the first at the bottom of the stairs. Silly man.
I would both hope and imagine they have learnt their lesson and hopefully others will have too. I've see the video of the buggy rolling onto the tracks but don't know if there is one of what led up to this. There may have been events that distracted the parents
I'm not saying it's right I'm just saying that sometimes it happens