From T-Mobile's website:

"FamilyMode helps you manage your family’s online content and time, it lets you set-up screen time limits, pause internet on demand or via schedule, locate family members and set content filters. The FamilyMode app is available for Android and iOS devices. Changes made in the middle of a billing cycle will result in full monthly charges of the new feature. FamilyMode customers on most T-Mobile wireless plans can also manage calling and messaging services by adding Family Allowances at no extra cost. Please contact Customer Care if you wish to add Family Allowances to your FamilyMode service."

Learn more about Family Mode here.