^^^ 24mm will work. Trek's BB90 size takes a standard gxp spindle, which is 24mm on the drive side and 22mm on the NDS. Your cranks would work fine with the OP's son's frame and existing bottom bracket.
Originally Posted by
rpenmanparker
Re-read the OP, he already has those cranks, he's looking for compact gearing, so something like this-
SRAM Force GXP Crankset - 2013 | TotalCycling.com
Or these-
http://www.bikewagon.com/sram-rival-...t-50-34t-black