If you have a family on the run, you probably grab a lot of quick meals at kitchen bar.
A counter bar is at the same height as the countertop (36 inches high). "The Rules" recommend giving each person 24 inches width and 15 inches depth minimum to eat comfortably. I'm not big on minimums, so I prefer to add 3 inches to both those because there’s often other stuff happening like surfing the web on your laptop or doing a bit of homework.
A bar-height counter is 42 inches. Again (as per comments above) keep in mind that you might want to carry out other activities. Just have a think about it before you finalize the dimensions and don't try to squeeze in too much.