How do I prevent my child from deleting her imessages?

My preteen is deleting her iMessages from friends, but keeping the ones from me and my fiance. Which has me thinking she's trying to hide stuff.

I tried to restore her deleted messages, but apparently there isn't any way to do that.. I'm hoping there is a way to prevent her from deleting her messages, then? Or is there any type of Parental Controls that is more in depth than the Restrictions under the settings?

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