Amtrak should have it set up so you choose your seat in advance. End of story. It would simplify everything.
For me, I like to bring my kids on trips. Sometimes 2 kids. Sometimes even 5 kids. Its always important to me to make sure we are all sitting together. Sure, "choosing a seat in advance" may solve that problem, but it also might mean that by the time I go to pick the seats, there will be nothing left but random aisle seats.
I don't count on the conductor to "let groups with children board first" since that almost never happens. I try to board at terminal points, and be first in line.
The best thing to do if you have 2 kids or 5 kids is have a Red Cap pre board your family group.
I don't think seat assignments will simplify everything. It will just create a different set of isues for people to whine about.