I am currently volunteering in the uk working with children through a volunteer organization called csv, more information can be found at csv.org.uk. They are not free but are a non-profit organization and you might find them cheaper than oterh volunteering organizations. Another extremely helpful resource is this book which has an A-Z list of volunteering orgs. in every country imaginable:

Worldwide Volunteering
By sir Richard Branson

Hope you find this helful and best of luck to you.