Well, I am no pro at this, so this is just my opinion. Whenever I have done a party (ok, so it was like Pampered Chef, scrapbooking, etc.), as a hostess, I usually get a free gift and then on top of that 10-15% of the sales in free items. You could go a higher % than that if you want (usually the % goes up depending on the total amount of sales).
BTW, the picture of your son is just beautiful.....as is your pics with your daughter--SO CUTE!!!