Personally, I wouldn't go with children during the evening hours. ACL isn't really a rough crowd type venue, but there's not that many kid oriented acts then and it will be overall safer for you.
I went to ACL for the first time in "07 and really enjoyed it. Last year, didn't like it as much. This year, ain't going. By nature, it's really oriented toward the alternative acts of which some are real good, but a lot seem to be of the overly introspective mopey need to get overthemselves type bands. Just wasn't in the mood for it this year. But, the weather forecast looks good and they've watered the holy shit out of Zilker to keep the dust down. Eat a chicken cone.