If you have added a DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages.
An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or Messaging features. An iPad with Cellular connectivity alone can only access IP-data services. An iPad is not, and can never be, a standalone substitute for an iPhone.
Unless paired with an iPhone, for FaceTime, you’ll need call using either an email address or AppleID. You will, of course, have set-up and AppleID for each of your children - and will not be sharing your own AppleID with them.
Make and receive FaceTime calls on iPad – Apple Support