I knew a mom who delivered meals through Meals on Wheels with her preschooler. She had only one child, though. She brought meals to the doors of various housebound people, usually elderly, and I believe interacted with them briefly at the door (i.e. she didn't just leave the meal outside the door). She had some apartment complexes, so there was some climbing of stairs involved.
I would think some homeless shelters locally might need hygenie kits (travel size toothbrush, toothpaste, comb, washcloth). The Red Cross might also need them for victims of fire or natural disaster. You might even combine it with a drive to collect the travel size items from friends or family who travel often or just want to contribute. A drive and the packing of kits would be pretty easy for the kids.
For Girl Scouts, a kindergartener would be put in Daisies. In my experience, that age group does not typically do tons of community service. I'd rely more on the other suggestions given here. I did know a 4th grader in Girl Scouts who did a range of informal service tasks, like walking a dog for an elderly neighbor. Raking leaves or shoveling snow are other neighborly tasks for someone sick, elderly, etc. However, those are jobs that would be mostly you doing the work with your 3 and 5 year old "helping."
Advice and commentary on living overseas
DD1 15, DD2 12, and DS 9