A select study panel recommended yesterday that the government and the private sector guarantee comprehensive health care to all pregnant women and children under 6 as the first step in a massive program to upgrade the nation's health. The immediate cost would be about $5 billion a year above current health outlays.

"Ten million children in this country have absolutely no health insurance whatever," declared Sen. Edward M. Kennedy (D-Mass.) as he introduced chairman Lisbeth Bamberger Schorr and other members of the Select Panel for the Promotion of Child Health at a Capitol Hill news conference. Kennedy is chairman of the Senate Health subcommittee, which backed the law establishing the study group two years ago.

The Schorr group, in a giant four-volume report, concluded that in the long run, the health needs of children and pregnant women would be served best by some form of national health insurance guaranteeing health care to everyone regardless of ability to pay.

But in the absence of such a program, it said, the first, most desperately needed, step would be to guarantee health care and family planning services to millions of uncovered pregnant women and children, a policy that would cost relatively little and would save large amounts in the future by averting care costs for people crippled and debilitated by infant and childhood disease. Many of these services are included in the pending Child Health Assurance bill, which apparently will die in the waning days of the congressional session.

The report also recommended: eventual expansion of these services to children up to age 18; large new programs to avoid accidents (a major killer and crippler of the young); nutrition improvements, including bigger programs to feed low-income people; better dental care and flouridation, and a step-up in health research.No cost estimates for the whole range of dozens of recommendations were given.

The panel said there are strong indications that preventive health outlays, especially for children, more than pay for themselves. Thus, it said, $180 million spent on a measles vaccination program from 1966 to 1974 was estimated to have saved $1.7 billion in future costs of retardation and deafness; genetic screening is estimated to save eight times its costs, and flouridation 50 times its costs.

The report said state Medicaid programs leave out at least 5 million children who meet federal poverty criteria, and millions of others aren't adequately covered by their parents' health policies. It said more than half of private insurance doesn't cover prenatal care for pregnant women and 19 states don't cover maternity care of women during their first pregnancy in the state Medicaid program.