My son just turned 5 years old. We invited our friends with their kids around his age. I organised that little kids' activities in the yard for them.
Few days before the party, my DP announced that his 19 year old son decided to come with his new gfriend (never met her). This year, SS only saw my son ONCE (his choice).
The evening before the party, my excited SD (15) announced to me that the SS decided to also bring HER new boyfriend (never met him).
When they finally arrived, they also brought the oldest SS (22) who sees my son ONCE per year (his choice).
Obviously, because they rarely see my son and SS wanted his gfriend not to be bored and get to know my son, they totally monopolized him. The 19 year old is very loud and likes being the center of attention, so he entertained my son inside the house, while the little friends were playing on their own outside, which was embarassing and he didn't touch any of the activities I put outside. I just couldn't get him out and before the party he was really looking forward to seeing his friends!
The oldest SS then started a chess game, so they monopolized the dining room table and I had to ask several times to stop it to do the cake (not to mention that it was hard to detach people from the game to eat, etc.). And of course, being older, they left quiet late, so my son ended up going to bed exhausted and even had a nightmare probably from being overwhelmed.
At the end, it seems that at least the oldest kids had fun. My other son is 15 and it was obviously more interesting for him than a little kid's party, except that the party was for my 5 year old, who had a good time too, but really not in a way expected and almost not playing with those who I invited. The kids might just decide not to come to his bday next year if he doesn't play with them.
I guess I just wanted to rant and see how to avoid this situation in the future, as I really didn't expect it and nobody asked for my opinion, yet I'm the one organizing my son's bday parties.