PARIS -- My first glimpse of the Children's Crusade that Jacques Cousteau has mounted to save Antarctica came when the letter from Margaret Thatcher's office crossed the doorstep. Imagine my new humility when I saw the addressee was not the foreign policy analyst in the family, but Lily Hoagland, age 10.

''Mrs. Thatcher has asked that your letter be passed to the Department with particular responsibility for the matter you raise,'' the prime minister's man wrote. The office manager of the leader of the opposition, Neil Kinnock, did no better. He sent along the address of another office to write. Not my job, said the two leading politicians in Britain.

They were responding to a letter my daughter and her classmates had written demanding to know what Thatcher and Kinnock were going to do about the Wellington Convention on Antarctica. Lily and thousands of other Paris schoolchildren were indirectly put up to lobbying the Iron Lady and the Leader of the Opposition by Cousteau, the explorer of the seas and now a major force in environmental politics.

Cousteau stops at nothing. In a truly subversive attack on the established order, he is mobilizing children to take on the bureaucrats and politicians who he says are squandering the habitability of the planet for the future generations. ''This is a powerful way of getting the parents' attention,'' the most popular public figure in France says with evident satisfaction as we talk in his small office near the Arc de Triomphe.

Wiry, erect and possessing a commanding manner much younger than his 80 years, Cousteau has already moved France's government away from its early support of opening up the Antarctic region for mining and other economic exploitation. He has also had impact on Australia and New Zealand. The Iron Lady, George Bush and others on the opposite side can expect to hear from him, and from the children who admire him, again.

To understand what Cousteau is about, first understand that he is in fundamental disagreement with ''green'' politics as they are practiced in France, West Germany and to a lesser extent Britain and the United States. He feels that those who are organizing ''green'' parties and entering governments on that label create a new and dangerous form of pollution: political pollution. ''To protect the environment, you have to work from the outside, not the inside.''

In West Germany and France, ''the ecology movement is being poisoned by becoming part of politics. Once it goes into the political system, ecology becomes the enemy of the other parties, instead of inspiring them from the outside as we do,'' Cousteau says in the most animated part of our long conversation.

Don't mistake these as woolly, idealistic murmurings from an old codger gone soft. There is cold calculation and determination behind the commandant's words. He has shown this in his effective opposition to the Wellington Convention, an international agreement that would in effect remove the controls on economic development of the Antarctic when the present 30-year treaty on the region runs out in 1991.

After reading that France was supporting the convention, he says, ''I decided to test public opinion on this. So I organized a petition, hoping to get 100,000 signatures. But it kept growing. When we got to 600,000, I got a letter from {French President Francois} Mitterrand asking, 'What's going on?' '' Cousteau told him, and he believes that Mitterrand has now committed France to blocking the opening up of Antarctica. To make sure, he reminds Mitterrand that the petition now has 1.6 million signatures and is still open.

In poll after poll in which the French are asked to rank the country's politicians, entertainment stars, athletes and other public figures, Cousteau scores way ahead of anyone else. ''People have seen me on television enough to know that I never lie to them. If I am careful about what I say about the greenhouse effect and the ozone layer, it is because we cannot yet be completely sure about what is happening. You have to maintain your credibility.''

In the United States, Cousteau has enlisted Sen. Albert Gore Jr. as an ally on Antarctica and has circulated 1,000 videocassettes to congressmen and their aides. The cassettes show Cousteau's recent trip to Antarctica, accompanied by schoolchildren from the six other continents. It was this cassette, shown here in my daughter's school, that captured her imagination and led to her letters to Britain's politicians.

''Today because of communications technology, children know as much as the wisest nobles knew in the past,'' Cousteau says as our conversation draws to a close. ''And adults know much more, of course. When people were illiterate, they had to elect the lawyer or the doctor or whoever had access to information and knowledge to represent them in government. But today the peasant has more information than the politicians, who lose their time in sterile partisan fighting. This kind of democracy is out of date.''

Margaret Thatcher's mailman should be thankful that school has just let out here. But come September, he can look forward to the next installment in Cousteau's Children's Crusade.