Mmmm, that's a hard one!
There are some health issues around children and the strength of insect repellants: http://www.rch.org.au/kidsinfo/factsheets.cfm?doc_id=7755 relating to DEET (http://en.wikipedia.org/wiki/DEET).
The best natural insect repellant seems to be: http://www.mosiguard.com.au/whatismosiguard.htm
We have the same issue for a trip to Burma at the end of the year. about 7 years ago