When adding a new child to your timeline, you will need to manage your contacts access/invite them.
Step 1: Tap on the menu button on your timeline (circled below):
Step 2: Tap on 'Manage Contacts'
Step 3: Tap on your family/friends name
Step 4: Tap on your child's name to give access
Step 5: Once you give access you can choose to enable 'Is this your partner?'