I think you could start at the beginning of any sub-series and do alright. The first couple books in each sub-series generally don't have any/much overlap with the other books.
Here is a visual map of that:
http://io9.com/how-to-read-terry-pra...e-h-1567312812
At that age, I would probably lean towards the City Watch books, but that really depends on your kids. If they do RPGs any of them would probably be fine but if they start with The Color of Magic/The Light Fantastic make sure they know that they get much better from there.