Hey there HaveSneakyKid,
There are a couple ways to help with text messaging on a child account. Although you cannot change settings to keep them from deleting messages, you can use communication safety options in Settings > Screen Time for your child. This resource will explain how that works: About communication safety in Messages - Apple Support
The other option would require being signed in with the same Apple ID, which will send the messages that are sent/received on the child's device to yours. Keep in mind, however, this will also cause messages sent and received on your device to be available on theirs as well. Messages in iCloud is explained more here: Access your messages on all your Apple devices - Apple Support
If you're looking for other options not mentioned or available, you can always submit feedback to Apple for future improvements: Product Feedback - Apple
Take care!