In my opinion, Cincinnati to Valdosta in late December is a bit aggressive, considering how early it gets dark, the holiday week traffic, and having kids along. I've certainly pushed that many miles in a day, and it's possible, but you end up being exhausted. However, Atlanta South certainly seems doable. I've stayed there before, and it was fine for an overnight...plenty of pull through sites and easy on/off the interstate. The only thing I recall was the bath houses not being great, but that was probably 8 years ago so it's quite possible they've been upgraded since then.
A friend of mine stops regularly at the Forsyth KOA on their way to the Fort, and they always have good things to say about it.
If you do decide to push through to the Valdosta area, I really like Eagles Roost in Lake Park GA for an overnight.
