Greetings Baba Ras Marcus, blessed love... I don't think anyone younger than their late teens should use herb at all. People's brains should be allowed to develope to adulthood without drugs of any kind (including drugs much more dangerous than cannabis that are routinely prescribed to children, such as ritalin).
Children, especially younger ones, should not be exposed to any kind of second-hand smoke either. Second-hand smoke is one of the possible causes of sudden infant death syndrome (crib death), doctors say they are not sure why this happens but second-hand smoke could be a factor. So you should not smoke anything anywhere near babies for sure.
I do know quite a few parents of youths who are now teenagers or young adults, parents who smoke cannabis, some moderately and some quite heavily, but their youths do not smoke or use cannabis at all. I know others who smoke cannabis with their grown children (young adults). I also am very close to the daughter of a [hard] drug-addicted mother, who used to smoke cannabis [and nothing else] from time to time to help deal with the stress she was living under. So there's a whole range of possible scenarios as far as parents, children and ganja. Could it have a negative effect on the youth? Certainly, especially if the parent is overdoing it to the extent that it affects his or her ability to carry out his or her parental responsibilities. But I do know a lot of people who smoke, eat and/or drink herb who I would consider to be very good parents and regardless of whether or not their kids grow up to smoke the occasional spliff or not, I feel they will grow or have grown into very intelligent, well-informed, sensitive, capable and reliable people.
I also know parents who smoke herb who I would not consider to be good parents, of course. For some of them their herb use/abuse could be the main thing stopping them from being good parents. For others it might be something else entirely and their herb use is neither here nor there.