I would think something outdoors at a park, splash-pad , in a parks and rec program, or your local YMCA would be most cost effective for that number of kids. Any of the play places/ McDonalds / Chuck.E. Cheese/ Build-a-Bear can all get very pricey with the more children you add, if cost is no object then go with what ever the birthday child is interested in.It would depend on the age of the birthday boy or girl along with the age range of the guests that you are inviting.
Doing a backyard barbeque and hiring a magician or clown and hanging a pinata could be fun too. Good luck whatever you choose just remember fun is the number one priority.
I was thinking of a place like Chuck E Cheese.. Kids love it there with the food and all the games.
A local park that has picnic area. Or your local fire dept. Or recreation center.
30 kids constitutes a large party therefore you need to have lots of space and activities. An Amusement Park will keep the kids entertained and provides lots of room for them to roam around. If you choose this option the kids need to be old enough to go off on their own or you need multiple chaperones to escort groups of kids to different rides.
Other ideas are the park, zoo, ice skating rink or roller rink. And the old standby Chuck E Cheese would work or an indoor playground like Gymboree or My Little Gym.
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