I need access to my child’s iPhone

Sorry, but this is no longer a technical issue, but a potential life threatening one. This would be a matter that you need to take up with your local law enforcement agency. No one here, nor Apple can assist you with accessing her phone if she is not willing to allow you to.

Only apps that are synced to iCloud, via her Apple ID, would be available from another Apple device logged in with that same ID. She may not have enabled syncing these other apps to prevent you from accessing them.

"Can't get into her phone" because?

Regardless, does she have her own "adult" Apple ID or shares one ID with you?

Is her phone now Activation Locked because too many attempts to enter its passcode failed?

Sorry, but this is getting a bit confusing. What does the iPad have to do with this issue? She has an iPad, in addition to an iPhone? Both are logged in with her Apple ID? ... and again, is she under 13?