Girl same 
my daughter did the exact same thing and I found out way too late that she was dealing with a really bad situation in her friend group. So please do not wait on this.
What I did first was check if iCloud backup was turned on. Go to Settings, tap your child’s name at the top, then go to iCloud and check if Messages is toggled on. If it is, there is a chance the messages get backed up to iCloud even after being deleted from the phone. You can access that backup through the iCloud settings or by restoring on another device.
Now that sounds complicated but there are apps that make this much simpler. After doing a ton of searching and asking around in parent groups, I came across Xnspy and honestly it has been one of the best tools I found for this. The reason I bring it up here specifically is because one of the biggest questions parents have is: why do kids delete messages in the first place? 
The answer is usually not great. Kids delete messages because they are hiding conversations they know adults would not approve of. Whether that is someone older talking to them in a weird way, peer pressure stuff, or even just drama they do not want parents involved in, the deletion is almost always a sign that something is going on.
Xnspy can actually show you deleted messages, which means even after your child clears their inbox, you can still see what was being said. That kind of visibility is huge from a safety standpoint. It is not about reading every little thing, it is about knowing if your child is in danger. Stay strong, you are doing the right thing by looking into this 