In April, a letter appeared in a Manhattan weekly, The Westsider, from an indignant mother. The only president we've got had recently visited the Hudson Guild Day Care Center, which was attended by the mother's 3 1/2-year-old daughter. On the day of Reagan's visit, the woman had outfitted her child with a T-shirt that said, "Stop Neutron Ron." The parent explained it was her way of protesting the president's preference for funding "bombs ahead of day care centers."

At the end of the day, when the tyke's father picked her up, he discovered that a member of the White House staff had insisted the child remove her T-shirt. The penalty for refusal was banishment from a group picture with the president. A teacher did take off the Tshirt, lest the little girl feel rejected. Her mother did not appreciate the gesture. "My daughter," she complained, "has had her first lesson in the workings of 'democracy.'

I wondered at the time about the parents. Turning a 3 1/2-year-old kid into an unwitting billboard did not strike me as the essence of participatory democracy. I also wondered, however, at the reaction of the White House staff accompanying the president. Even a 3 1/2-year-old has the right to wear a talking T-shirt. True, the Constitution does not guarantee her the right to be photographed in the sunshine of the president's smile. But giving her that ultimatum -- ditch the shirt or stand in the corner -- showed no White House class. Or common sense. A picture showing the president benignly countenancing innocent dissent would have done him no harm. Quite the opposite.

Since April, loyal White House staff have gone much further to make sure the president is not disturbed by dissidence when he visits among us. They also do not want the television audience for these appearances to be disturbed by any signs that the president is not beloved by all of his people -- except for the professional politicians and union bosses on the other side. How can we deal with the Russians if our people are not as united as theirs?

So, when Reagan decided to bless Cincinnati with a visit on Aug. 20, thought police were stationed at metal detector centers guarding Fountain Square, a traditional First Amendment forum for any citizen with a revelation or a beef. At the checkpoints, all signs and banners were taken away, with particular attention to those expressing something other than devotion to the president. Once people were past the barriers, bright new signs were handed out to one and all. Each of the approved signs celebrated our great good fortune in having Ronald Reagan as our leader for four more years ("I Love You, Ron").

A 16-year-old, Anna Batsakes, had lovingly made a sign tht said simply -- in red spray paint on a bedsheet -- "Save the Trees." It was seized by the guards. (She may be too young to remember that the president once accused trees of polluting the environment.)

In any case, the president, in Fountain Square, was able to gaze on such unbroken affirmation of his policies and his person as to probably make him wonder if he might indeed be God's instrument for this nation at this fateful time.

Some of the demonstrators who had been stripped of their visual political speech were both angry and somewhat apprehensive. Said one, "I think they (Republican officials) did something the KGB would have been proud of."

The White House at first said the confiscation of the signs had been ordered by the Secret Service for security reasons. Not so, said the Secret Service; they only took signs that might be used as a weapon or in which a weapon could be concealed. They were not responsible for ordering the wholesale censoring of the signs. Entering the argument, White Huse spokesman Larry Speakes offered this unique interpretation of the First Amendment in a public place: "It was our rally, we were staging it, and we had the permit."

Meanwhile, the American Civil Liberties Union of Cincinnati, among others, has filed a suit in federal court to bar this sort of Czechoslovakian behavior at future rallies. The lead defendant is the Reagan-Bush '84 Committee.

Elsewhere during the campaign so far, in Decatur, Ill., and San Jose, Calif., bearers of signs that might disturb the president have been cordoned off at a decent distance from the rally. In Austin, Texas, as in Cincinnati, all signs not approved by thought police have been taken away from demonstrators at security checkpoints.

If the president is elected by acclamation, he may never again have to face the possibility of being disturbed. At Fountain Square, and at all the other political playing fields in our great land, we may well see him henceforth only on a giant screen. Below, demonstrators will be allowed to carry any signs they like, but it will hardly seem worth the effort to wave an unkind message to that giant, smiling face on the giant screen.