My experience is, if you go through the preschool, you have to invite everyone. That's if you're putting invitations in cubbies. That's as it should be. If you have a parent list, you can invite whomever you want. Our guideline was always "However old you are, that's how many guests you get" - it's a great system! 5 years old, 5 guests. I agree with everyone else to keep it simple. These elaborate theme-based parties are expensive, overdone, and hard to keep up with. I never wanted my kid to have 30 gifts, and I never wanted to have 30 kids in my yard or local play place or a park
Be aware that a lot of parents are starting to question parties in McDonald's or other fast food places (food quality, obesity issues). Pinatas are fun but put things besides candy in there. We have a number of diabetic kids in our neighborhood, and parties/trick-or-treating are a nightmare for them. We did some simple theme parties - when he was into dinosaurs, we did a dino party and decorated the deck with his dines. We bought plastic Easter eggs (discounts soon!) and put little toys or candies in them, then hid them around the yard. Those "finds" became their substitute for "goody bags". The year he was into "bugs" we made a "caterpillar" out of cupcakes and decorated them in outlandish fashion, gave the kids fake bugs and little magnifying glasses for a "bug-hunter" kit to take home. In some areas, there are bowling alleys that put bumpers in the gutters so that every rolled ball makes it down the alley safely with no gutter balls. In New England, we have candlNatureepins, which use a smaller ball that little hands can handle. Local zoos and museums have deals too - sometimes the kids get to go "backstage" to see animal care. areas often have marked trails, and you can give the kids a list of natural formations to find - often the local nature organization (Audubon, etc.) does that work for you.
Whatever you do, go small and simple! The kids don't care about all the stuff that parents seem to get sucked into. They just want to run around, find "treasure", throw balls or whatever. The other parents will thank you for not going overboard and making them think they have to do the same!