RE: Best choice for 6 year-old w/ newbie parents
^^^ +1
You can get the brushless for about $275 shipped brand new off of ebay. (That's where I got mine and I have no problems.) Out of the box with the supplied battery, it will do about 30mph and in your case, the best thing is that there is a "beginner mode," which allows you to limit the throttle to only 50%.
It comes with everything you need...BUT, only one battery and a few hour charger=a bummer for most people. A decent charger (will charge each battery in less than an hour) will cost you no more than $40 for this application and a few more batteries will cost you no more than $30 a battery.
Oh, and it can take two batteries in series to go 50+mph...crazy stuff and wheelies will be very easy.
My mini E-Revo brushless, 6 1700mah batteries, and y-harness cost about $450. I already had a charger for my other trucks, so I didn't need one.
For you, I would say that 30mph would be more than enough for a beginner, so the truck would cost $275, a few more batteries would cost about $60, and a charger $40, so you'd be looking at less than $400...double what you want, but I do not think you can get anything else that has the potential and great features like this truck.
By the way...you're a cool mom! [8D]