Where are you, 'Jack's' parents? Don't get me wrong, this boy is quite nice, but I have never met you and you have never met me, yet you think it's fine for Jack to have a sleepover at my house even though I don't even have your contact details?
My 9-yr-old has his mate from school sleeping over, and an extra kid has turned up expecting to participate in the party. This kid has his sleeping bag and toothbrush but I've never met him or his parents before and I don't have any contact details. What happens if he has an allergic reaction to something or wants to go home or breaks an arm? Or, seriously, what if I'm just not a good person for this kid to be around? How would they know? Geez some people are weird.