Look at the 735,000 Americans who are homeless and what do you see? For one thing, plenty of children. At least 100,000 youngsters under 18 are homeless on any given night, and that doesn't include those who have run away or been kicked out by their parents.

For another, you'll see a decrease in the number of housing units for low-income people - 2.5 million fewer than in 1980.Those are just a few of the more pertinent figures from a report released recently by the highly respected National Academy of Sciences.

The report should help ease the apprehensions of city officials who fear that providing more shelters would only attract more homeless people to their communities.

The great majority of homeless, the report notes, are long-term residents of the city where they live. What causes homeless people to move to another city is the prospect for work, not the prospect for welfare.

Another important point: A look at homeless families with children across the nation shows many more adult women than men. Homeless men on their own outnumber homeless women by a long shot, of course, but there are considerably more women with children.

Even if these women find jobs, most will never be able to buy a home. Which leaves the option of renting. Or does it? Scripps Howard News Service reports that an estimated 25 percent of rental units nationally do not allow children. And as many as half restrict entry to families with children.

Which brings up a little-noticed piece of major legislation passed by Congress this year and signed by President Reagan.

The Civil Rights Act of 1968 has been extended to protect families with young children. In other words, just as it has been illegal for decades to discriminate on the basis of race, sex, or national origin, it now is illegal to discriminate against families with children.

The amendments also protect some older people from the unwelcome intrusion of noisy children. Apartment complexes designed for the elderly will remain exempt from the requirement to admit children (that includes all retirement communities), as will buildings in which at least 80 percent of the units are occupied by persons over 55.

Even so, the majority of apartments fall under the new law.

Steve Ira, president of the National Apartment Association, calls the law "pretty drastic" and predicts rents will rise as a result. Maybe he's right, but only time will tell.

Nevertheless, in an era when more than half of poor people live in single-parent households, which are usually headed by a woman, it only makes sense to open options to families with children.

Otherwise, a restricted market will propel an increasing number of families to look to government or private charity to supply them with housing.