If you want health insurance protection for more than one person you must buy a family plan. It doesn't matter whether you are an elderly retiree and spouse, a government worker with a spouse and several children or a single parent with children. All of you are a family.

Buying family coverage can be tricky because the needs of the larger group (your family) can be varied because of wide age differences.

If you have small children, you may want a plan that pays most or all of the cost of "well baby" exams.

Many families with children prefer traditional fee-for-service plans. They allow you to pick the doctor. But families also should check to see whether their doctor is affiliated with a local health maintenance organization. HMOs usually offer lower premiums than regular fee-for-service plans.

HMOs can be good for people planning to have a family because many offer full maternity coverage with no deductibles. But most HMOs limit your choice of doctors, hospitals and sometimes treatment.

Walton Francis, author of Checkbook's "Health Insurance Plans for Federal Employees," a guide to the current insurance open season, says HMOs provide good to excellent well-baby coverage. He also recommends you look at fee-for-service plans such as GEHA American Postal Workers Union, Blue Cross standard and the National Association of Letter Carriers. His book ($5.95) is available from the Center for the Study of Services, 806 15th Street NW, or by calling 202-347-7283.

Checkbook rates health plans by your total estimated annual cost. That includes premiums, membership dues or fees if any, plus anything you may have to pay out-of-pocket. Yesterday we carried Checkbook's ratings for singles. Later, we will rate plans for retirees, people with special medical problems and for workers, such as FBI agents, Capitol Hill employees and Foreign Service staffers, who have their own health plans.

Here are Checkbook's family-of-three 1991 cost estimates for fee-for-service plans:

Blue Cross standard option nonpostal family premium would be $920. Checkbook estimates out of pocket expenses to you at $1,130, making your total cost next year $2,050.

APWU premiums and dues, $1,070; out-of-pocket expenses, $1,060. Your likely total cost next year would be $2,130.

Mail Handlers high option premium and dues, $880; out-of-pocket expenses estimated at $1,370 for a 1991 total to you of $2,250.

Postmasters high option premiums-dues, $4,520; out-of-pocket estimate, $1,150, total cost to you estimated at $5,670.

Blue Cross high option premium, $5,200; out-of-pocket, $1,110, total cost to you, $6,310.

Alliance high option premium, $6,490; estimated out-of-pocket, $1,200, for a total cost to you next year of about $7,690.