I went ahead and registered him. He is 8 so it is 400 meters--very doable. Website says they are 95% full for the kids races. The Kids' Mile is totally full.
I think he will have a good time. Apparently spectators pay $12 to enter the sports complex, but we are leaning toward adding on waterpark fun & more anyway, so we'll see.
The kids get a t-shirt, goodie bag and medal, so I think the $10 registration fee was worth it.