the .22Mag is slightly longer and has more velocity. I would reconsider the choice of calliber though ... I realize the .22 WRM is leagal up hear ( I'm from Maine too) but there are far superior choices out there.
Not knowing your daughter size I can only speculate but ... .44 mag, .30-30, .243 Win, 7mm-08.
NEF makes a youth model in the .243 W and that's what I started my son out with this last season. good luck to you and your daughter.