are you saying that the 2400 lipo is like running an old 1500 stick battery?
Several people gave good replys about the 2400 Lipo
This little pack is awesome, and a great entry to lipo pack with a really good price.
But, (similar to an old 1500 stick pack) it has limited run time (But probably a lot better run time than those old 1500 packs)
The SLASH is a big, heavy vehicle. Depending on how it's being treated during bashing, it's probably getting a lot of stop/start type running.
My guess it you should get 6-8 minutes of play/run time per charge safely (possibly more) but something VERY important to know, learn and pass on to your child is IF YOU FEEL THE POWER SLOWING DOWN - STOP RUNNING THE VEHICLE
With Nimh and NiCad batteries you could run until a vehicle STOPPED moving. Do this with a LIPO and it could be ruined, or at the very least it could severly hurt the future performance of the battery.
I hope all works out great, as the slash is a lot of fun, and LIPOs are so much better (Long RUN) than NiMh batteries are. Something to consider for future is using a large mAh pack (5000 mAh) which will more than double the run/play time. Or purchase a couple more of the 2400's, and charge them all up ahead of time...run one 5-6 minutes and change them out.
Sounds like you've made some good strides, and learned quite a bit in a short time.