Conquerors through Christ recommends Covenant Eyes’ screen accountability software. Parental control content blockers are a good start, but Christian accountability is extremely important in every struggle too. Covenant Eyes helps provide enable that accountability.
Do not overreact. This was going to happen eventually. Nothing is ruined. And your child is normal. It’s not shocking that this is happening, it’s shocking that you didn’t know about it. Now that you do know about it, you can have a conversation about it with your child. Let them know that sex is good, but pornography is not. Let them know what your plan will be going forward. Consider putting a content blocker on the devices your child uses. Let them know they are forgiven and loved.