I believe there is very little experience here with treatment of children.
You may want to try a post on:
http://www.medhelp.org/forum_disclaimers/show/340
and other forums dealing with children.
I do not have any personal knowledge with the forum link about, so I send just as something to try.
Best of luck, and I assume your doctor would not prescribe a medication for a child if it was know to stunt growth... I also believe 2.5 mg is a very low dose for adults and may be a typical dose for children, that is small more sensitive bodies.