Think of the inverse, someone who doesn’t care about where they sit save money.

On the other hand, I never understood this obsession with grown people acting like it’s the end of the world if they don’t sit together. My wife and I fly a lot together - over a dozen trips this year - and she flies more frequently by herself. We both prefer window seats. We hardly ever sit together unless we can get 2 seats next to us by ourselves like on larger planes with a 3-2-3 combination or exit row seats in main.

In this particular case, it lets the parents trade childcare responsibilities back and forth during the flight, which can be a serious boon on a long flight or if one of them starts feeling unwell.