My son has been driving a Stock Slash for 2+ years, and is 7 now. He also drives around my Slash 4x4 on 50% lately as his truck needs a motor.
I would say a brushless Slash 4x4 on 50% power is the same speed as a brushed slash on 100% power, both running a 7 cell battery.
(Traxxas speed controls you can cut the power by 50% for younger drivers)
I would also recommend the 2.4Ghz version of any vehicle for 3 reasons.
1) Far less interference or glitchiness. Turning on the truck first before the radio will not give you a runaway vehicle typically.
2) Only needs 4 AA batteries instead of 8 for the standard transmitter radio, and lasts longer. Transmitter is also lighter as well for kids.
3) Short stubby antenna on the transmitter. A 3 foot telescoping antenna will not last long with a younger child. They either swing it all over the place, or bend break it trying to put it away.
JMO....