Airlines absolutely need to make sure parents and small kids get sat together. It doesn't matter how - charge families more, whatever - they just need to figure it out and fast. Reasons in order of seriousness least to most:

1. Comfort of other passengers
I have sat next to/near all manner of talkative, squirmy, kicky, tantrum-prone, noisy, crying, messy, first-time plane panicky little kids. I cannot imagine sitting next to such a child without its parent present. It would be dreadful - tomato sauce on my purse, wailing for mommy, who knows. Passengers will absolutely have to play parent to these kids, from opening juice boxes to comforting them when their ears hurt. Do you all fly business and therefore cannot imagine this scenario?

2. Sexual harassment liability
The airlines are already in the news for sexual harassment on planes and flight attendants who aren't trained to deal with it. (Like not moving the passenger, continuing to serve alcohol to the harasser, not calling the cops when landing.) Passengers are getting crammed in closer and closer together and imagine if a kid is forced to ride the whole way next to a molester separated their parent and not knowing what to do.

3. Emergency liability
If there's any sort of emergency or reason to deplane, and a child is sat far behind his/her parent, the first thing the parent is going to do is attempt to barrel down the aisle to the child (parental instinct!), thus blocking others from escaping. Kids on their own could also panic in an emergency and ignore other passengers' attempts to get them to deplane. Or even have a panic attack and fight off attempts by another passenger to help putting on an oxygen mask. If a child dies in a situation that is deemed to have been preventable if the parent was nearby, the lawsuits and public condemnation would be astounding.

The airlines shrugged off the David Dao incident because they know that customers don't really have a choice and are a captive audience. However, the reputation/risk management teams need to get on this and recognize that #2 and #3 above would be an absolute catastrophe and just make sure it doesn't happen, period.