FAMILY WHO WANT TO VOLUNTEER WORK (Chiang Mai)

We are an Aussie/ British family, kids aged 6 and 7, and would like to do some volunteer work somewhere in Thailand for 12 months.
Maybe work with children, school or orphanage etc and/or do some building work though we are open to other options.
We would just like to give back to a community somewhere but need to do it where the cost of living isnt so high. Would appreciate any info or advice on this from those of you already living there.
Thanks for your time.