How do I set up the iPhone to see my child's text .

If they have iMessage setup you can enable Test Message Forwarding to another device (I've done this with my Mac). See here: How to forward SMS/MMS texts on your iPhone - Apple Support. This will cause your child's iPhone to forward to your other device. Note however that your child could turn this off on their iPhone - I don't see anything in restrictions to disable this. Restrictions can stop them from turning off iMessage though - so you'd be able to always see those on the other device.

Also be aware that with the future iMessage in the cloud coming when you read a text on the other device it may mark the message read on the iPhone letting your child know someone has read it.

One way you can see their calls and texts is to have their Apple ID on your phone

This is part of iCloud and you share iMessages and Calls

When someone calls him or sends him an iMessage it will go through to your phone as well

but also the other way around