Agree that the 1 and 3 yr old may be a challenge...I'd focus on the same type of games they enjoy at home. We have a 7 and 9 yr old. Ladderball is always a hit as are bikes and scooters. Various travel-size games...Trouble, Sorry, any games you play at home. Both of our kids like uno and other card games as well as Farkle and a couple other dice games. We keep a couple of different size round balls and footballs too.
Nature scavenger hunts are always a hit with all ages too. Put the list on a paper bag (for carrying the treasures) and cater the difficulty to the age. Just make sure it's all simple stuff and be aware of applicable park rules about "collecting" stuff. :B
