We have a rucksack with ponchos, camera, lens, suncream, and sunhats in and then my smaller bag with park tickets, room keys, money, hand gel (ds can't wash his hands in water because he gets a rash!) and ADR numbers in it.
DS has a small disney bag he carries with his sun hat, hand gel, autograph book & pen and his camera.
With children that young I'd say your essentials would be spare clothes, nappies, wipes, drinks, sun cream and sun hats. I got so irritated at all the irresponsible parents not putting hats or cream on their children - we were so in the minority.