i see no problem, but 'god' is usually capitalized... if you don't feel right about it, simply substitute 'oh, gosh!' or 'oh, wow!' or any of the many other alternatives that kids would say...
<chuckles lightly> Yeah, most ten year olds these days have much worse in their vocabulary than "Oh, my God!" Parents wish that was the worst they'd say. Now, a six year old? Yeah, may not want to include "Oh, my God!" in a book directed to them. (I could be wrong, though...)