I manage a volunteer center. Your best bet for youth that young are as some mentioned: libraries, churches, animal rescue groups. Another good bet is a museums or historical mansions. Historical programs are much more willing to work with youth than other programs. Lastly, try the naturalist or eco-friendly organizations in your town. Sometimes those groups are willing to have youth represent for them and/or help during special events (fairs/festivals).

You will mostly have a bad time placing a youth with any agency that deals with a vulnerable population.

Also, another tip many parents don't realize. More agencies will be willing to work with your young child if you can volunteer WITH them! Good luck!