Equipment changes...about the only way you will know, if the airline doesn't tell you, is to continually check your flights. But, that's kind of a pain to do.
The thing that bothers me the most is the lack of concern on the airline employees' part. They evidently showed no concern for those kids. I'm sure there had to have been a few people that might have been willing to change their seats in order to have those kids seated with a parent. And to have to sit with a 2 y/o on your lap, after paying for a seat, is just intolerable.
United needs to look into how their gate people are trained and what they are doing...or, in this case, not doing.