For DD's 2nd birthday party, I got the Honest Kids things. The age range/number of kids was about the same as you're talking about. I made extra sippy cups available if people needed them, but for the most part, people have their own on hand.
I would be okay with cheese pizza. If it's a neighborhood party, it's not out of line to suggest that people bring a dish to share. I would also have some other snack-type foods on hand. You didn't say when the party was, but if it's at night, I would go easy on the sweets, since many kids will already have candy and the only thing worse than having your kid all hopped up on sugar is your kid having a meltdown because everyone else gets a cupcake.