I'm in the same boat as you are, except I have two girls so I have to buy THREE of everything, and it all has to be the same so no one has and advantage. I also recommend Traxxas, mainly because of the easy availablity of parts from my LHS. I bought my Rustlers used off eBay to save money and I try to do the same with upgrade parts, but as you will find out with kids, you'll need a reliable and quick source of replacement parts to limit down time, which keeps everyone happy.

A lot of people recommend the Slash, but I'd go with a Rustler XL-5, mainly because it's about $40 cheaper ($80 for you), and the Rustler is supposedly a better all around vehicle than the Slash. Anthoer benefit of the Rustler XL-5 is that the new models are waterproof/water resistant, which will allow you to run in damp or wet conditions.

I'm sure the Duratrax is a great car, but from what I've seen, Traxxas is a more "universal" car, whether some consider it good or bad; they even sell their stuff at PepBoys.