Now and again you may need to reinstall the FamilyTime application on your child's handset. This could be because they have a new phone, or they have carried out a factory reset and deleted the application. Other reasons for doing a reinstall are that the phone needed repairs, or had the firmware upgraded. Whatever the reason, you shouldn't need to reinstall the application very often.
You'll then get taken through the installation process as you were when you first installed. Use the same email address and password, and the phone will automatically be linked to your existing account for that phone number, with all the existing security settings.
Unfortunately if your child changes their device, you will need to set the device up from scratch.