Personally, I wouldn't make much issue of it. Once the children have a routine they expect their snack. It's just like us (adults) we have our meals routines we expect to eat at the same time, poor dears, they do not have control wether their parents come earlier or not. What I do is explain this to their parents so they have the choise of bringing snacks and go, or wait until snack time is over or wait while I get their child's snack ready to go, my goodness a child smile is priceless. Parents go happy so the child and I feel I did my good work which is also priceless. I had parents that had to take the bus so I'll make sure their child's snack is ready to go.