In the U.S.A, the Federal Fair Housing Act of 1988 makes it illegal to discriminate in the sale or leasing of housing to several protected classes of people. "Familial Status" is one such class. This status is defined as anyone who is the parent (or custodian) of a minor child, and is even extended to pregnant women. The only exceptions to this law are made for a handful of "seniors only" apartments who meet certain rigorous tests.
Real Estate Licensing Laws in most (if not all) states require Agents and Brokers to understand the Law and pass a test demonstrating this understanding. Most reputable property management firms provide training for their leasing agents as well. The penalties for violating Fair Housing Laws are stiff and can result in a fines, punitive damages, and revocation of licenses. If you feel you have been discriminated against because of familial status you should contact your local H.U.D. office immediately.
Also, "steering" a prospective lessor or buyer to a particular home or apartment for any number of reasons is a form of discrimination. Parents of children cannot be shown only apartments in certain buildings or even encouraged solely to considered them. Nor can children be unreasonably denied the use of the property's amenities such as the pool, clubhouse, or laundry room. Discrimination of people with children usually translated into discrimination against women and minorities. Those days, thankfully, are no more.