Unless there's something you aren't telling me about usage, the SMS standard is firm at 160 7 bit characters for all U.S. providers, at least those I am aware of. Perhaps the limits you've describe refer to IM or email in this particular situation(?)

http://en.wikipedia.org/wiki/SMS

If it's a Sprint branded phone, I suggest you visit the Sprint sub-forum @ howardforums.com for specifics on what's going on w/ your kid's handset.

http://www.howardforums.com/forumdisplay.php/53-Sprint