The first thing to do is to make a minor investment. You will need 16-ounce paper cups, packages of several different flower seeds, and some potting soil. Make sure the children are equipped with hand sized digging tools.
Into each paper cup put about 8 ounces of soil. Then put in a few seeds from each packet. Mix gently.
Load the wagon with the seed mixture, tools, and some jugs of water or a watering can.
Go with the children and begin knocking on doors. Maybe that grandmotherly type at the end of the block is a good place to begin. Explain that you would like to plant a small flower garden for her. Allow the grandmotherly type to choose the location.
The children can then dig up the chosen spot and plant the seeds. Make sure to water them as you would in your own garden.
Clean up any mess and move to the next person. Another good suggestion is to have the children offer to set out the person's trash or do some other light chore before you leave.
These little gardens will have to be watered and weeded just like your flower garden. As a matter of fact allow the children to plant their own little gardens too.
The children will tend these little gardens all summer. They will get to know their neighbors and learn how good it feels to do a good deed. They might even decide to adopt the grandmotherly type. It will benefit everyone.