Your original post was a little unclear so hopefully this will answer your question. Installing a new ROM or rooting shouldn't be necessary if you only want to be able to use the device to send and receive text messages. One way to do this is to sign up for a free Google Voice number. It's a fantastic tool Google provides where you get your own personal number, though it will probably be from some strange area code. Then, you can install the Google Voice app and use it to send and receive text messages using your Google Voice number. If your daughter's Nexus 7 has cellular data, she should be able to send/receive messages as long as she has signal bars. Otherwise, she needs to be in range of WiFi.
Just a side note, the Google Voice number can also be used to send and receive calls if a VoIP app is installed as well. I use GV Phone and so far have liked it. While using a Nexus 7 as a phone will probably be very unwieldy, it may give you some piece of mind knowing your daughter has one more way of getting in touch with you should she need to.