He's going to be 3! What to serve for "brunch" at park?
I love making a deconstructed Nicoise Salad for afternoon parties. Arrange all of the components separately on one large platter. Use good tuna jarred in olive oil, steamed small red potatoes, capers, olives, dressed lettuce, steamed haricots verts, sliced hard-boiled egg and whatever else you want. Have dijon mustard, oil and vinegar, and a couple baguettes available too. You can also have a cold beef tenderloin, sliced and arranged on a platter with a horseradish sauce. Lots of assorted crudite and a platter of roasted peppers, mozzarella, tomatoes and olives. Everything can be done ahead of time and eaten at room temp or cold. These are foods that kids like too. For the salad, you can bring everything in baggies and arrange the platter when you arrive. It looks very festive for a party.
Stick with finger food and presentations that can easily be covered if the weather turns wet or blustery. Mini-quiches are fine warm or cold, bake in muffin tins and/or mini-muffin tins. Raw vegetables and dip - if there will be a major kiddie contingent, put a dollop of dip in a plastic or paper cup and add veggies cut in sticks. Pigs in blankets.
Muffins and cupcakes - for neatness, pipe frosting into the cupcakes rather than putting it on top.
I know it might sound a little low-brow, but maybe you could do some bagel pizzas for the kids... slice bagels in half (scoop a little of the insides if desired, spread with a little tomato or marinara sauce and sprinkle with cheese and bake. They work at room temp, and will be popular with the kids. You could also do some baked chicken tenders for the kids as well.
I'd do a nice big fruit salad, as that goes over with kids and adults alike.
You could also do some nice petit four size sandwiches in flavors popular for both grown-ups and kids.
Assuming you're not going to a park where there might be a BBQ, where you could grill some burgers and dogs?
Mediterranean foods do well at room temp - mezzes like hummus, baba ghanoush, pita, dolmas, greek salad...