You have a right to the quiet use and enjoyment of your apartment.
If other tenants are interfering with that right, you should report it to the management, in writing.
The use of medical marijuana is allowed in California, so the manager may be correct that there is nothing he can do to stop it.
You may be able to reach an agreement with the other tenants that they not smoke around your kids, or try to keep the smoke indoors as much as possible. You may ask your landlord to move you to a different unit, or let you out of your lease.
Your rights are the same as if you were smelling cigarette smoke all the time. You have a right to bring an action for nuisance against your neighbors and your landlord. A nuisance is anything that is unsafe or offensive that interferes with the quiet enjoyment of your apartment.
The problem is, I have litigated this issue. Even though I had medical testimony that the marijuana smoke was dangerous for my client, a lung cancer victim, the Court (at least this particular Judge) ruled that because the tenants had a right to smoke their "medicine" in their apartment, the landlord was discriminating against my disabled client, and not responsible for creating (or stopping) the dangerous condition. I think that Judge was wrong, but your landlord is correct that this is a difficult legal problem, and it has yet to be sorted out by the courts in this state.
If the smokers are in a public area, this is an easier question, but that is probably not the case at your complex.
This might be a good time to look for a single family dwelling or cottage.