This can be done with any grade level - kids love making their own concoctions. It can take a bit of time, so ask the staff if you could work this into their day of celebration. Double duty - you're not at home counting out 20 bags of 100! ;)
100 Day Trail Mix
First - supply the kids with plastic gloves to reinforce safe food care handling/hygiene. Supply a small variety of snacks - popcorn, cereal squares, pretzel, goldfish, cheerios, raisins, dried cranberries, banana chips are a few ideas to start - then have the children pick out 100 pieces (10 or 20 of each for the little ones is easy) and place them in their own snack/treat bags. You can sneak a host of mini lessons in there as well, i.e. shape recognition, sorting, patterns. It can be a lot of fun!
Oh yeah - and if time is of the essence, you can make the muffins or something like that and present them in the shape of "100" on a serving tray. Mini bagels can also be the zeros, while a carrot stick, celery stick, pretzel stick or the like can be the one. Let us know how it goes!