Unlike some other mobile service providers 48 don't support group or family accounts for managing multiple SIMs, accounts, memberships, subscriptions etc. under one login.
Every 48 user needs their own individual account/login.
To get your daughter set up you need to use a different email address and go through the same process that you originally went through to get your son set up on 48.
See the New to 48 FAQs:
If you need to contact 48 support about this then see the links in my signature below.
