If I want to host a birthday party here will I need to call and reserve a spot ? Or is it first come first serve ?

It's first come first serve

You aren’t able to reserve this park through the city, it’s only on a first come first serve basis

I would recommend to get the permit

I would advise you to call in advance because if you do, it will be reserved in your name, so you are guranteed to have your party in the location of your choice

It is better to call in advance because someone may have already put there decrations down and already had everything set up. So you can try but it is better to call in advance