nitrosportsandrunner gives you a great 3s pack. it's a monster and your boys may have too much on their hands, don't get me wrong it will handle it, but the motor will run hot at that pont so gearing down or dropping the motor timeing (programed at the factory at 15degrees) this is done with a programing card, or with the button on the side of the speed control.
if they want POWAH give them that pack, you may have a little more laundry to do when they get back. it becomes a 1/10 savae flux on 3s lipo (trust me i've fixed them enough, someone shapped thiers in half, many broken bumpers and several hubs, keep in mind we sell hundreds of these a year)
my two packs of choice are:
the gens ace has more runt time and power. but the skylipo is more than enough. i've owned mine for 5years. now it's nitro cousin is also siting on my shelf