That's a good question.
The leaves of the cannabis plant are not typically consumed by someone who wants to get high. It's the flowers that are rich in THC and other cannabinoids. The reason they're not consumed by people interested in getting high is that they contain very low levels of cannabinoids. So the classic sort of pot leaf that you see on people's T-shirts and things like that doesn't have a lot of THC or CBD cannabidiol in it.
With regard to the impact on children—and I'm a plant scientist, not a medical doctor—I'm going out on a limb a little bit, but just on the basis of the plant chemistry, if kids eat cannabis leaves or a small cannabis plant, not a lot of cannabinoids are going to get into their system and affect them.
On the other hand, if the plant is flowering and it is that THC-rich material or CBD-rich material, they could ingest it and receive a dose of that. It gets a little technical here. The plant actually doesn't make THC. It makes an acidic form of THC. So tetrahydrocannabinolic acid is the form the plant makes and you actually have to heat it to form THC, which is why smoking a joint or baking a brownie or something is required.
If a child were to eat the raw bud of cannabis, they'd get mainly the acidic form, which is non-psychoactive. The fresh material is not capable of getting you high. You need to bake it or heat it or smoke it to get there.