If your SIM is not registering on the network firstly check that the SIM is inserted and has been inserted correctly (turned the right way). Now re-start the phone.
If the problem persists check to see if the child has received any SMS from the carrier requiring any type of action: You can do so by going to your parent app. Click the profile picture of the child on the dashboard and then choose Activities. If any SMS has been received it will show up here.