Sounds good. Maybe the point right before their meeting so the two are introduced to us.. Mayhe the day before his birthday, to make it like.. Extra sad. His parents are maybe decorating the living room for the upcoming day when your demon gets there. Out of curiosity for what his parents are doing he goes down stairs and then sees what has happened to them. 
I'd like to start of at 8 or 9 so the kid is still easily influences but can speak for himself and do things. I might go as low as 6 years though.