With the help of a 7-year-old girl and the specter of the U.S. Central Intelligence Agency, the Philippines' first election campaign in five years of martial law has gotten off to a lively start.

President Ferdinand Marcos still rules by decree and the April 7 interim National Assembly election results will not change that. But Marcos, by allowing his most celebrated opponent, jailed former Sen. Benigno "Ninoy" Aquino Jr., to enter the assembly race, has put some of the old fire back into Philippine politics and opened his administration to great opportunities and dangers.

Aquino, 45, confined to a room at a military prison has won wide sympathy and foreign press attention by putting his youngest child on the stump. Usually dressed in slacks and T-shirt, the little girl stands on a chair before rapt audiences and says: "I am Kris Aquino. I am 7 years old. My father is Ninoy Aquino and I have not been able to live with him for a long time. Please help me to get him free."

Candidates on Marcos' slate for the 200-member assembly have sought to counter Aquino's emotional campaign with an emphasis on their experience in government. Several government candidates, including Foreingn Minister Carlos P. Romulo, appeared at a press conference yesterday and handled questions on weighty matters such as the negotiations over a U.S. bases treaty.

So far, Kris Aquino has appeared at 13 rallies, about one a day since the campaign began. "Of all my five children, it is she who takes best to politics . . . She has absolutely no stage fright," said Aquino's wife, Cory, in a telephone interview. Aquino, once considered a likely successor to Marcos as president, was arrested on charges of murder, subversion and arms possession the day Marcos delared martial law in September 1972.

Marcos has not overlooked sympathy for family ties in his role as manager of the pro-government slate of candidates. Under the rules, he automatically receives a seat in the assembly. He has accused the Aquino-led "Laban" (Flight) group of distributing leaflets making personal attacks on his wife, Imeida and even hinting that he is not the father of his eldest daughter, Imee. Imelda Marcos, governor of greater Minila, is leading a pro-government slate against Aquino's group in Manila, the only place the opposition has organized an anti-govenment slate.

This week the combatants have begun to aim at a favorite target of politics in the former American colony - the CIA. Marcos' defense secretary, Juan Ponce Enrile, an assembly candidate, charged on television that Aquino once confessed to an army officer at the prison that he had links to the CIA.

"My husband was apparently joking with the man but he didn't get the joke," said Cory Aquino. In a letter to Marcos, Aquino said he had been "made a victim of 'quotations' lifted out of context from electronically monitored conversations that were eighter garbled in transcription . . . or were deliberately distorted to suit the particular line of attack of my adversaries."

There have been reports for years of CIA. involvement with important Philippine officials, both those supporting and opposing Marcos. Knowledgeable sources say it is likely that many such reports have some truth, but the government has so far failed to produce any solid evidence of Aquino's involvement.

Marcos has agreed to allow Aquino to answer the charges in a televised interview. The date and time have yet to be set and Aquino is demanding certain conditions to guarantee that he will reach as many people as saw Enrile make the original charges.

The opposition slate has little hope of victory April 7, although some preminary polls suggest Aquino himself might have a chance of winning an assembly seat. Whether Marcos would allow Aquino to leave prison then is unclear. The charges against him, which seem to rest on very shaky evidence, are still being reviewed by a military tribunal and by the supreme court.

The election provides Marcos with a clear opportunity to show off his martial law regime as enlightened and democratic. So far, the campaign seems restrained in comparison with the usual Philippine standards of payoffs and shootings. The opposition however, has been able to get unusual publicity for the attacks on martial law - the principal reason they have decided to contest what would appear to be a hopeless election.

Marcos is arguing that his rule can help develop the economy without the disruptions of the wild and woolly democratic past. The opposition argues that the economic advantages are not worth the loss of personal freedom. The way in which the campaign develops and ends may set the ground rules for what is sure to be a long period of conflict between the two sides.