Facebook is a tool. I don’t friend people I don’t know.
Its possible to use Facebook improperly. Its possible to use it properly.
Be aware if your child says “I’ll friend you” they can still block you from seeing a lot. So if you let them have it, make sure you have access to the password.
Another consideration is that if you say no, they may do it behind your back. Some family kids did that and lets just say…they didn’t use facebook properly. Their parents just dug their head into the sand and ignored the internet and their kids using it for a long time. Luckily, they grew up and have outgrown most of that.