It's interesting you were excused for that reason. Or maybe I misunderstood and you weren't actually excused but rather was able to reschedule?
Interestingly on our County's website they have a specific question and answer on that:
"Are mothers with children at home excused from jury service?
No, there is no automatic exclusion. If we were to exclude all mothers with small children, a significant portion of the population base would be excluded from jury service. This would result in juries that are not truly representative of our community. You are responsible for arranging for your own child care if you are called for jury duty. If the scheduling of your jury service is the problem, you may reschedule your jury service one time only by contacting the jury coordinator"
So I guess in my County's case you can't reschedule because you have a child but you can reschedule one time if you are unable to make it work as far as child care.