I have the solution for long-standing problem families with small children who need (or perceive the need) to sit together, and all the hassles that occur when they do not sit together.

Offer a "Family Convenience Package" when a passenger 5 or under is listed in the travelling party. Either make the package free or make it a bargain compared to seat selection for the whole party.

This package offers seat selection in the last X rows of the aircraft for the entire party and a "pre-boarding" option. This is all indicated on their boarding pass.

The pre-boarding announcement is then changed to "We are now boarding passengers holding Family Convenience boarding passes."

The people who need more time will be settling out of the way of other priority passengers likely near the front of the plane. At deboarding time they don't clog the aisle. They have assigned seats so there is less of a need for begging and pleading for seat switching.