If you're travelling with young children being seated together isn't a luxury, so it's basically a tax on travelling with children, and a fairly expensive one ($100 easily for a return flight perhaps for four seats?) when you've paid it for all the seats for your family.

Though when we had young children, we seriously considered not paying and enjoying having somebody else looking after our four or five year old for the flight :-)

Given it is a necessity, I feel it should either be a compulsory extra cost if you have children below a certain age or it should (ideally) be free to be seated together, so that people who do pay for particular seats know that there won't be an unsupervised child allocated to the seat next to them.