It may seem a little generic, but I had my girls' birhtday party at Chuck E Cheese. One was turning 3 and one was turning 4 and we just had a joint party. We were able to have it during the week (in the early evening) and hardly anyone was there. The kids can pretty much just run around in the play area as it is nicely closed off. The front door in also closed off, and an employee checks each child and parent at the door for security purposes. The staff does everything--set up, serving food and cake and clean up. The kids even at this age love to play the token games. I booked it on line and was given extra tokens. All in all it was a fun day and the girls asked to have their next party there:) I think the total cost was under $200 for food (for parents too), drinks, cake, goodie bags and tokens. We went to the location at Kooser and Blossom Hill in San Jose but Ihear there is a new one in Cupertino on Stevens Creek.
Also, there are lots of nice city parks that have BBQ and play areas and you can reserve your tables. You have to provide all the set up, food and clean up-as well as arrange activities. You also have to worry about weather depending on the month of the party.
Anyway, I have been to many park parties for friends' children and they were lots of fun too.
Good Luck.
H.--mom to two 4 year old girls and a 20 month old boy in San Jose