I am thinking about having my daughters 5th birthday party here. She wants to be able to swim and I would like to do a big picnic with all our family. Does any one know if I need to plan this in advance with the park or can we just go the day of?

Yes, I would go early. Especially if this party is going to take place on a Saturday. If Saturday, all parties should get there early to get a good parking spot. However, the pool will be crowded on a weekend.

You can just go on the day but go as early as possible so you can claim a good spot/tables. They have every and any amenity you'll need but again, go early, at least your immediate family to set up and your guests can come later.