Does it have a picnic area for a party and would I need to reserve?

Yes anytime you are going to put a jumper or "water slide" in a public park you need a permit. A regular picnic / party no

Its a birthday party.... Do i need to get a permit for a water slide jumper?

Yes it does have a picnic area. From past events my family has done there all you need is to get there early unless it's going to be a large function then yes, you do need to reserve it