I rode an awesome (but older) Titus Racer X with v-brakes and never had a problem braking. They were also much less fussy than disc brakes and I never had the rubbing issues that I have had on two mountain bikes with disc brakes.

Caveat: I am under 120 lbs. and try not to ride in the rain or in muddy conditions, like TigerMom pointed out above.

The brakes are likely one worry you should just take off the table. The main thing is to get her comfortable with shifting and riding in general.