Only send out as many invites as you can manage. We usually end up with every child invited coming. If you send them out early enough and you get some no responses, you could always send out a second wave.
I need some help as I'm organising my DD birthday party. She's 4 and it's the first time I'm inviting some of her friends. I was wondering: how many kids do you invite and how many kids turn up? If I want to have a 10 kids, how many shall I invite? and how do you send reminders?