I've been to a few in my day. Some are great, some suck. Here are some tips to make sure yours is good for young and old:
1. Offer Food right away
I didn't have time to eat lunch to make it to your party called for noon. Feed me, damn it. Then, maybe I'll be able to calm my kid (and myself) down.
2. Booze.. now.
This may be a personal preference, but I think it is appropriate for kids' parties. It's still a party with adults present. I will be much happier to make small talk with the other guys who are just as unhappy to be there as me if I have a beer in hand.
3. Communal activity first, then groups, then free play.
Kids don't always mingle easy first. Make them sit next to each other at least. Coloring, art & crafts, something where they do it independently but at the same time. Then, they'll be warmed up to play as a group. Musical chairs, freeze dancing, that sort of thing. After that, let them go crazy and form their own groups. They'll be able to meet kids thee genuinely like and start friendships.
4. Paper plate and plastic cups
Don't give me a slice of pizza on a ceramic plate. You won't get it back.
5. Balloons for all
They love these things. I don't know why. Same goes for stickers. Have enough for every kids x2. In every color.
6. Music in background
At the least, it will not make a silent room feel so awkward. At best, it will drown out the inevitable tantrum that some kid will have when there aren't enough balloons.
7. Leave an area for adults to sit and talk.
If the Moms & Dads have fun, the party will be more fun for everyone.
8. Let there be light, not dark
Kids don't like the dark, so make sure it's in a room with a lot of like. The back room of a local bar that resembles a cave will not work.
9. Know your audience.
Do not have a party on a Sunday during football season. Don't have it on Halloween night. Don't make me miss something I would rather be doing.
Those are my tips. I hope the party I go to tomorrow follows these guidelines. But of course, it's on a Sunday. See rule 9.