That's what I thought... I know the stampede came with a planetary; wasn't 100% sure about the rustler... I do remember Thorp had a ball diff for the rustler (oi, now that's a bit of history, lol). But Thorp went the way o' the dinos, if I'm not mistaken....
I'm going to ammend the earlier post of mine; I agree with smaxxin... The idler gear is the probable culprit. Even if it isn't, if you put anything hotter than a 20 turn motor in there, you will garenteed strip the idler. Save some future heartache, and pony up for the alum version.
And while you have the trans open, check out the diff for wear and tear.
Also, and this one is optional; if you plan on letting the kids have a faster motor; you might want to consider the slipper clutch hopup (if it doesn't have one already). That will allow the trans to slip a little, and prevent any other gears from being chewed up.
Otherwise, excellent choice of truck for a kid; Rustlers take a heck of a beating and come back for more. I sold several at the shop I worked for, and every one of 'em went to a satisfied customer.