iPads are capable of iMessaging (sending messages to other iOS device users or Mac users). Go to Settings>Messages and sign into iMessages.
By themselves, iPads are not capable of regular text messaging (SMS). In order to do any SMS messaging, they need to be paired with an iPhone. Does your child have an iPhone? If so, with iMessage activated on both devices with the same Apple ID, go to Settings>Messages>Text Forwarding (on the iPhone) and enable forwarding to the iPad. If your child does not have an iPad, you'll need to search the App Store for an app that will send SMS messages using the iPad's data or WiFi connection.