M.C.
You could contact the Samaritan Inn in McKinney and see what kind of help they need. You might be able to volunteer to help the kids there with holiday crafts, storytime, or something like that, and then your youngest could participate in the activity, if not technically volunteer. Another idea would be the McKinney food pantry--4 and 7 year olds are old enough to sort food, prepare packages (one jar of peanut butter in each bag...) etc. I'm pretty sure the Samaritan Inn is the only homeless shelter in Collin County, but there may be a food pantry closer to you that you could check out. Kudos to you for getting your sons involved in service so early!