rat lung worm is extremely rare, however there has been a recent (in the last 10yrs) emergence of it, particluarly in the Sydney area. I remember reading an artical about it in that's life. Chances are if he plays with them he will be fine, but rat lung worm is a very real concern even if rare. I encourage my kids to 'look but not touch', not because of the risk of rat lung worm as such, but more because they need to leave little creepy crawlies alone in general. I guess it is a bit like everything, it all have the potiential to be harmful (potting mix is much more likely to be harmful then snails for example) but you just have to weigh up the risks, teach good hygeine and hope that you and your children will be with the majority of the rest of the population who have no ill effects.

I remember playing with slugs and snails all the time as a kid, and picking them off the vege garden with mum. My brother used to eat them. yuck.