Hayneedle via Google Express has the Razor MX650 on sale for $305 + tax ($100 max discount with code EXTRA25).
This is a highly rated electric bike that is rarely on sales below $350.
if you know your way around RC stuff you can upgrade the motor to the 1800 watts brushless motor which can be found on ebay.
YouTube has several videos showing how to modify this bike. Regardless, this is a great starter bike out the box for your kids.