Cranks for Haro Backtrail X2

[harobacktrailx2]

hi everyone...i'm giving a 2004 haro backtrail x2 nyquist to an 8-yr old kid as his first real bmx bike. He's tall enough to ride with the seat almost all the way down, but the stock one-piece cranks on the bike are way to long to ride in that seat configuration. How do I determine the proper aftermarket crank type and size for the backtrail X2 so I can order a new set? Do I need a new sprocket too, or is the old one (remember one-piece crank) reusable with new cranks? And if I need a new sprocket, must it have the same number teeth as old sprocket to work with the existing freewheel hub? I can't find any specs online--not even at harobikes.com, nor do any of the parts houses I can find online list parts for bike of that make/model/age. Thanks a lot for your help.