I don't think you should have to invite the friends and parents of your other children. Cha Cha and Gator each have their own groups of friends who are closer in age to them. I wouldn't invite Gator's friends to Cha Cha's birthday party and vice versa. And as your children get older, I think that will be even more true.
I wouldn't invite them.
struggling mom to Gator ('03) Cha ('05) Sisi and Greenbean ('07) and the hubby child ('71)
"I'm sure we can handle this situation maturely, just like the responsible adults that we are. Isn't that right, Mr. Poopy Pants?" - The Naked Gun 2 1/2