How can a child without a phone text on an iPad?

That depends on who and how he wants to text.

iPads can send iMessages to other Apple devices or use any number of messaging Apps such Facebook Messenger Kids, Signal, Telegram, Just Talk Kids etc.... to message users of those apps.

If by "text" you mean SMS messages to non-Apple devices such as Android Phones, then the iPad cannot do that without a paired iPhone.

If exchanging messages with Android or non-Apple devices, you’ll need to use Cellular SMS/MMS messaging.

An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services. If you have added an eSIM or physical DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages. An iPad is not, and can never be, a standalone substitute for an iPhone.

An iPad can, however, natively exchange messages with users of other Apple devices using the iPad's Messages App.

Set up Messages on iPad - Apple Support

Send and reply to messages on iPad - Apple Support

Other proprietary messaging Apps are available from the Apple App Store. A popular alternative messaging App/platform, that is also not available for iPad, is WhatsApp.