5 Year Old Bday Need Help With Wording!!!!

Hi,
I am at a loss as to the polite way to "exclude" younger siblings to my son`s birthday party. He is 5 and will be having a birthday at a place that charges $25 per child. We are also limited to 15 children and the place does not recommend children under 5 but will charge for any child once the 15 max is met. Most of our friends have more than one child and usually everyone is invited. Because this is a costly party and a limit to the amount of children and the recomended age I want to put something in the invitation without sounding RUDE! Can anyone help?

PS: It`s a Harry Potter themed party at a magic shop with a magic show that is 60 min long.