In order to use Teen Time ecosystem you need to setup Teen Time on parent (your) device and Teen Time for Kids on your child’s devices. There is no limit on a number of parents and kids in each family, but there can be only 1 device added for each parent due to security concerns.
Please note, any changes you do in Teen Time app will reach your children’s devices within few minutes and requires mobile internet or Wi-Fi connection available. If the connection is not available when you did your changes, settings will be delivered on kids devices as soon as they will go online.