Consenting adults not harming others should have the right to poison themselves as they see fit.
The question you need to ask yourself isn't whether or not your teenage kids - soon to become independent adults - will try weed. The question is, do you want them to be educated about potential harms and get ID'd until they turn 21, or risk them getting an illegal product that isn't tested from a violence-ridden cartel (that doesn't check IDs, by the way)?
I highly recommend that you educate yourself about this drug, as I'm not sure of what scale of harmfulness cannabis can be classified as a "hard drug" [1]. The New York Times' 2014 editorial board's writeup on this issue is thoroughly documented and excellent [2].
[0] https://www.drugabuse.gov/publications/drugfacts/high-school...
[1] https://en.wikipedia.org/wiki/Drug_harmfulness#/media/File:2...
[2] http://www.nytimes.com/interactive/2014/07/27/opinion/sunday...
People get addicted to drugs mainly because they self-medicate for anxiety or depression, that's what you should really worry about actually, concerning your children.
I am going to assume that you grew up in a country where alcohol was available to teenagers, to a greater or lesser extent. And yet you also - presumably - managed to avoid turning in to a raging alcoholic.
Despite the availability of drugs and alcohol, you managed to avoid becoming a slave to either of them.
The mere availability of drugs (of any kind) will not immediately turn your children in to stoners, alcoholics and junkies. The causes of addiction are deeper than that.
As you said, you know what it is like to be a teenager. Teenagers experiment with various things that adults would prefer they avoided. Given that fact, let me ask you this:
Assume your children are going to drink underage (it is statistically likely that they will). Would you prefer them to drink a) a bottle of beer that an older friend purchased on their behalf, or b) a jar of moonshine, which was homemade without any quality control, and might contain adulterants?
This is not a rhetorical question, or a snarky one - I'm genuinely curious as to which you would prefer.
> My point is that I don't want my kids being turned into stoners and that the law should be on MY side, not the guy pushing drugs on them.
You seem to be saying that your children will be turned in to stoners if cannabis is easily available to them. Given that alcohol will be available to them, do you worry that they will turn in to alcoholics?
If not, why not?
If so, are you also campaigning to return to the days of alcohol prohibition?
If weed is forbidden/taboo, there's a good chance that your child will come across a successful/popular/intelligent peer that completely discredits everything you've ever told them about "stoners". My fiance is in medical school at a highly selective school and from interacting with a lot of her peers socially I've been again amazed at how many regular smokers are highly intelligent, driven and successful. Once it's truly out in the open, these successes can have a lot more context and we'll truly be able to study and discuss what truth there is to the stoner stereotype.
Should it? It isn't on your side if they want to (as consenting adults): ride a motorcycle, drink alcohol, go heliskiing, visit a strip club, buy condoms, or go bungie jumping, for example.
Should all potentially harmful activities of any kind be criminalized as well, because you have kids and the law should be on your side?
EDIT, for clarity: But no one who advocates legalization is advocating the scenario you describe. Total fantasy.