Well, I have seen a couple with kids spending far more on drinks (and perhaps food) than a group of college kids splitting a pitcher 4-5 ways to eat for free/cheap at happy hour.
I would look at it a different way (of course). If the adults with kids are regular customers, and it is a relatively family-friendly place (some pubs are), they are welcome to bring their well behaved children and use a table. If they just show up to grub on the food, then I have some issues. I have the same issues with people over 21 who only show up during that time to nurse a draft and chow down.
Also, I know you used to be stuck in the kitchen, but bars regularly charge $2 and more for a glass of pop/soda/soft drink. It's sometimes cheaper to drink beer in a bar. Don't simply assume the bar isn't making $ from a table because someone under age is sitting at the table.
But I can only semi-relate to your problem as I sit at the bar when I go to a bar.