Hi magenta6651591 !
This link, which is a T-Mobile Support page, should answer your question on where to manage Family Allowance. There is a magenta coloured button, which I highly recommend you bookmark on your browser, as well as directions on how to manage your F.A. on the T-Mobile app.
Any questions? Come back here and I'll try to help, or call 611 from your T-Mobile device. Thanks!
Manage your T-Mobile account