Before and after school care can get old fast, because you quit working outside of the home so you could spend time with YOUR kids. Anytime you have someone else's kid in your care, they get more attention than your own. It may work to take in your children's friends before and after school though. Just be sure that it doesn't conflict with whatever after school activities your kids do. Watching a young one during the day may prove to be fun for you, but be sure to pay attention to what age you liked best. You may not be able to get anything else done all day long, and your house will start looking like it did when yours were small (crayons on the wall, juice spilled on the couch, toys all over). No matter what age you choose, be sure that you have permission and the ability to tote little one where ever you want to go. It is very frustrating to be stuck at home not able to do anything because you don't have permission to leave with someone else's child.
I personally found it much more rewarding to just have my own and focus on them, but mine are 1 and 6, so it is completely different.