Having a birthday party at a park means there is plenty of room for your guests and that there is built-in entertainment for kids with the playground equipment. There are some things to consider before the party, though.
Check with your local Parks and Recreation Department to see if you will need to reserve any special areas of the park that you want to use. Special areas that may need reservations include barbecue pits or grills, tables and covered pavilions. Usually reservations are free.
Next, since you are having an outdoor party you will want to check the long-term forecast for your area. A great place to get this is the Weather Channel.
Also, make sure that your decorations are appropriate for the area. Streamers often will blow down in the slightest of winds and table cloths need to be weighted to keep them in place.