I did address your concerns, sort of...
First of all, if - as I contend - cannabis is in fact not as bad for you as alcohol, tobacco and other presently legal drugs, why should you be any more concerned that your children decide to indulge in it than if they decided they'd give ciggies a go? Decriminalising cannabis will no more encourage your children to use it than to take up smoking, and your efforts to dissuade them to should be in proportion to your efforts to dissuade them from using any other permissable drug.
You'll note that I am not suggesting you allow your children to use illegal drugs. You seem quite hung up on the idea that your children should not be encouraged to consider an illegal drug as 'OK' - that would not, plainly, be the case if use of the drug were decriminalised.
Secondly, you mention that we don't know what the long term effects of cannabis use are because its use has been illegal. The two propositions don't follow, though.
Simply because the use of cannabis has been leglislated against in this country does not mean (a) that it hasn't been used, nor that (b) it's been illegal to use it in any other country. Its use has, in fact, been perfectly legal in Holland, as you probably know, for some time, and the whole reason for the current debate is because it's been used here for many years as well.
So far, no known long term serious damaging effects, beyond those otherwise associated with drugs such as alcohol or tobacco.
Although I cannot confirm the fact, you may be quite right in your recollection that the driver of the train (or at least one of them) involved in the Clapham rail disaster had smoked pot in the 24 hours before the day. Has a connection been made between the two events that I don't know about, then?