Get lots of sand toys and sand trucks for the small ones, I never put kids on the swings as it just takes too much of my attention. Bring snacks like arrow root cookies to distract if needed. LoL
Your age group was mine last summer. The baby's had only one section of the park they could use so I sat right infront of that area. Most kids I find don't wander to far from you, especially if they know you have treats.
I always went super early to the park around 9am so it was never busy. If it was too busy we just went for a walk.