Thanks for all your replies.
I've had a bit of a poke about on the interwebs. Lots of places don't take children under 14 but a few do.
Projects Abroad has family volunteering for families with children 3 and over. Most of the other organisations that take younger children were based in the US or UK and for the sake of ease I would rather use an Australian based company.
Since it will be a while until we can afford it I guess I have plenty of time to find other options.

I just realised the no environmental stuff might look a bit dodgy. I'm not a climate change denier or anything, it's just that they are rather spoilt and entitled little people and it would do them good to see how the other half lives.