10k front, 7k rear.
Never go off whether someone else has shimmed "X" product or not, every diff/gear mesh should be checked individually and dealt with accordingly.
Don't forget to change out the pinion to compensate for the lower diff ratio. I would shoot for a 35-40mph top speed using those Nimh's.
It's an awesome thing you are doing for the boys, hopefully they let you on the bmx track!