If your 3yr old's attentions span is the same as my 3yr old, the 6-cell pack used in the Mini-quake is just about perfect!!!!!
If you are using the stock NiCAD pack, upgrade to another 6-cell nimh pack, you will get longer runtimes!!!!
Not to sell you here...but look at our website, we offer a couple of pack options that can be a direct drop in NiMH solution for the Mini Quake (btw, is my favorite truck right now)