M.F.
The Woman to Woman pregnancy resource center in Denton is always looking for help in their resale shop called Twice as Nice (sorting donations, cleaning, etc.). It's on loop 288 in Denton, north of McKinney street.
Also, there are likely opportunities in your own local church.
Cumberland Presbyterian Children's Home (www.cpch.org) has one workday each month where volunteers come help with their grounds -- little kids could water plants.
Check out the "volunteer connection" link on the United Way of Denton County website at www.unitedwaydenton.org. Many of the opportunities listed will say what kind of volunteer it's appropriate for -- groups, individuals, adults, youth, etc. You can likely tell from the description of each opportunity whether it would be appropriate for your children to come along. If you can't tell, there's usually a contact number.
You could even come up with your own way to help a local organization and propose it. For example, Cumberland Children's home also provides housing and counseling for a handful of single parent families. However, they do not supply household needs like they do for the children who live there without their parents. Maybe there's an opportunity for you and your kids to provide a meal, or Friday snacks, or something...and the kids could help prepare the meal/snack, then go with you to drop it off.
And, there are some neat opportunities for local missions if you look at http://www.thevillagechurch.net/adults/missions/comm_miss...