Yes, we survived! 20 children and 20 adults in the heat at Lake Accotink.
I managed to get there early enough (10:15 am) to get a "close" parking spot and got most everything setup by myself while watching two children. In the meantime, DH picked up the balloon order and the cake. The pizzas arrived on time (12:15 pm) , the candles would not light with the matches (or the slight breeze) but we sang Happy Birthday anyway. Then it was time for some miniature golf and a ride on the carousel. The kids loved the pinata (a pull-type pinata -- no "whacking" Curious George!) and I tossed a lot of candy around for the little ones who don't like the "squish effect" of kids diving for candy from the pinata. At 2 pm it was time for our boat ride on Lake Accotink, where Natalie got to drive the boat! It was really hot (95+ degrees) so we shortened the boat ride to 30 minutes and head back to our picnic spot (under tents) to give out goody bags and say "thanks for coming".
DH and I cleaned up quickly and we were all home by 3:45 pm. Nicholas fell asleep on the (short) ride home and napped until 5:15 pm. Natalie had no nap and was a wreck by the early evening -- she was SO tired and just outright wired.
As she said early on in the party, "No more outside birthday parties for me!"
9/10/06 ETA: Proof that Nicholas got into the cake early on in the party: