Sorry, but I don't agree this is an issue the airlines should be held accountable for (except when they reassign seats). IMHO, if you are traveling with a child or someone else you "need" to be with, then it is your responsibility to take the steps necessary to ensure this happens. I would never play roulette with my young family members by not selecting seating prior to arrival. And the concept of forcing someone out of their "paid" seat so that they can be together is a bit on the entitled side (again IMHO).
I pay to have an isle seat in the forward section of the plane so that I am in the final boarding groups (more time in the lounge area waiting with only an under seat carry on) but still in the seating section which I prefer. When I have been approached to move I have offered to accommodate if they compensate me for the price I paid to have that seat. This has always been met with disdain by someone who obviously cared less for my needs/wants than their own.
On a different perspective, I have given up my seat when there were people with physical or mental challenges that needed to be accommodated. There was a lady flying in to a medical appointment in Dallas whose travel companion could not get a seat next to her due to the late booking. I gladly exchanged seats because it was just the right thing to do and I have done so several times.
A simple solution could be for the airlines to impose mandatory group seat selection when an itinerary includes young children. This would eliminate the roulette factor of seat assignment at the gate and ensure young children are not separated by rows. After all, we are simply paying for a service and if you need an obvious additional level of service, perhaps the minimum fare should reflect that. Or the airlines could simply increase all fares so that seat selection was included in each ticket as it was before, should you choose to take advantage of it. This will not likely happen as the competitive nature of air travel drives the airlines to find new ways to reduce the base price yet recover those costs through add on fees.