It's a great
idea, can't wait to see them try to put it into practise though.
I moved around a fair bit as a kid, but one place I lived the kids (alright, me too) were drinking Southern Comfort on the weekend at 11, and some kids were getting stoned regularly at that age too (not me, didn't start that until at least 15). Can't see ciggies being any different, if the kids want em they'll get em.
btw - times have changed but the first time I went out drinking I was 14, and I sat up at a bar in the mall at surfers paradise with a 16yr old mate and got absolutely mangled on many things, finishing with 4 or 5 tequilas.
My point in sharing this is - when you're a kid you say to yourself "adults are so stupid, so out of touch, they don't know 1/4 of the **** I get up to, don't become one of them when you're that age,
REMEMBER what it's like, be cool. " Kids grow up fast, we did, and we're here talking about it.

How to prevent them starting smoking? I don't have a good answer to that.
My dad cleaned my room out when he found out I was smoking, I came home and all my stuff was in the dirty stinkin wheelie bin. Didn't stop me smoking, so I would suggest that route doesn't work.