Just say no, without guilt or remorse.

When planning travel, families should budget airfare just like lodging, meals and activities. This means selecting fares that allow everyone in the party to sit together. Airfares are at all time lows (adjusted for inflation), airline credit cards bring additional savings (free checked bags, companion passes). If the incremental cost to reserve seats in advance exceeds the budget, then consider another location, shorter trip, or leave the kids home with family - my wife and I tended to favor the latter when our kids were young.

What's next: families demanding free upgrades to minivans or premium SUVs when renting sub-compact cars? 2 bedroom suites when booking a standard room with one double bed?

I had a situation where a couple with a "lap child" (kid was definitely over 2 years old) somehow talked there way at the gate in to first class on a flight KOA-SAN a few years back, mother attempted to bully me in to moving to row 1 - bulkhead - not going to happen ("you were a kid once"). Prior to departure, I spoke with the ground supervisor, voiced my concern, mother refused the offered of an empty row (3 adjacent seats) in Y+. Kid was disruptive during the entire flight, miserable experience for the parents - I enjoyed 3 movies on the tablet, wearing noise reduction headphones. Wrote about the experience to Alaska Listens, received a voucher worth approximately half of my first class ticket.