From a very early age both my wife & i spoke with our children about weed & other drugs,in those days the police routinely visited schools with a ride along self proclaimed drug expert,they'd give talks to the kids about the serious dangers of smoking mj,then they'd tell them that if they knew anybody smoking weed to tell them & they would help them get better,by the thousands children across the nation were turning in uncles,aunts,brothers,sisters & even their own parents.
We explained early on exactly what mj was capable of & what information being fed to them was false,also we never hid our mj use from them nor did we lie to them about why we were smoking it.
The next level of talks came when they were teenagers,they allready had a good understanding of what mj was so we took it a step further & expressed our wish that they remain drug & alcohol free until they were finished with school,that included college,the reasons we gave them were not fear tactics but honest reasons like needing to focus on getting smart so they could have their choice of good paying jobs instead of spending decades involved in back breaking manual labor positions.
We also explained the stigma that would follow them throught their schooling & entry level job hunting should they have any hint of mj useage on their permanent records.
Our talks always finished with us stating that once they finished school we would support their choice to smoke if they wished but until then it should be off limits at our request,once our children entered colege we talked more & I explained to them not to get fuked up with party time in school,to keep their eye on the education that i busted my ass saving & paying for.
I laid some guilt trips on them also,my health has been painfull for the last 25 years due to a broken back earlier in life & they knew i hurt constantly,putting them through school pushed my retirement back 7 years to pay the tuitions & i asked them to please not make me waste 7 years because they couldnt wait to party,our intent on sending them to college was to break the cycle of manual labor in our family & it was sucessfull.
Our children turned out exactly as we could have wanted,both have tried mj on numerous occasions & still use it when the mood strikes but they are not daily or even monthly users,they have the same relationship with alcohlo,every once in a while they let loose but its a very small part of their lives.
So my answer is yes,tell them not to smoke,explain to them whats important in life,dont hide it from them,instead teach them about it & how to responsibily use it & other substances such as alcohol,dont let them get their information from tv,friends,school or the street,teach them & instill a knowlege base about the drug that they can draw from when its their turn to decide if it is going to be a part of their adult lives.