Gearing, Voltage, KV - OH MY!

I recently got a Duratrax Raze for a great deal, it has a RCM motor mount and battery tray. I want to build it up for my 5 year old son. He does pretty good with his brushless slash on 2s but I sometimes limit his top speed with EPA's. I know Castle does not recommend this but it seems when I gear it very low, he spins the tires too easy, and I dont think less voltage is possible.

I have a spare Mamba Monster ESC that I want to use in the buggy. I dont yet have a motor or battery for him. Can you guys recommend a motor/battery/gearing setup that will keep temps down and top speed under 30 (probably under 25 is better for the 5 year old)? I dont want to jeopardize the ESC.... as I have had my share of problems with the MMM. In my buggy, with MMM/2650, on my small backyard track, 2s (4000 25c) on 15/48 is perfect...temps are low, power is perfect, speed is perfect. But I fried an ESC and when I contacted Castle, they were concerned with me running 'only 2s'. so now I only run 4s, even on my little track and havn't had a problem since.

I just want a reliable setup for my son to practice with. Any suggestions on a low top speed setup?