I'm going to let my son's friends smoke weed at our house
That doesn't sound wise to me. Are you sure you want to let your son's friend do something illegal?
I just want them to have a good time.
It is wrong to be ok with kids doing drugs in your home. You should not let them do it but also not report them.
Oh well no big deal, we will just have some 420 bros here at my home.
You should be aware that since it's your home, you are the one responsible for what they do in your house. If someone were to find out that you were letting minors do drugs in your home you could be charged by the police.