Legal Question in Landlord & Tenant Law in California

Before going down the road of legal remedies, I would recommend contacting the owner(s) of the building directly and see if they will do anything about it. If not, you can escallate it by informing the owners that they are liable for the manager's conduct, including his/her management of his/her children, and that if they are not stopped from interfering with your mother's right to quiet use and enjoyment of her leasehold you will have to take legal action. Now bear in mind that "quiet" is a technical legal term that does not have the same meaning that an elderly mother would attach to it. The legal standard is essentially the same as nusiance, meaning the children have to be so disruptive as to effectively deprive your mother of the use of her unit.