Mom, Dad, 3 year old, 2 year old, 7 month old. No problem pre-boarding 3 flights over past 10 days. Had they denied us pre-boarding, rest assured we would have taken our sweet time to sit in our seats and it would have been a vengence delay. I found all the UA GAs to be great, not the same for the ticket agents or phone people in India.
I think the reason that you have never been denied is that you have a real need for it. The problem with an established preboard-with-children policy is that it will be abused.