Hi Aaron723, thank you for posting in Apple Support Communities.
If I can make the first suggestion or confirm that your 5 Year Old has his/her own AppleID and the following link will explain in more detail to do this:
https://support.apple.com/en-us/HT204976
If this is the case, then can you look at the following link which explains how to setup FaceTime on the iPad for your 5 Year Old:
https://support.apple.com/en-au/guide/ipad/ipad0997e101/ipados
In regards to the data plan for the iPad, whilst it has a number, that number cannot be used as a number for FaceTime which is why you are not able to connect the number to FaceTime. Only mobile phone numbers can be used for FaceTime. iPad SIM cards are not mobile phone numbers as they are Data SIM cards only.
Therefore, as long as your 5 Year Old has his/her own AppleID and that AppleID is setup on the iPad you can then contact him/her via FaceTime through the AppleID.