A VALUABLE AND necessary effort to repeal a provision of Maryland law permitting the killing of children went down in defeat last week. The Maryland state Senate voted 26 to 21 against a measure that would have prohibited the execution of minors convicted of first-degree murder. No one of any age has been executed in Maryland since 1961, but the death penalty was reinstituted in 1978 and the first execution under the new statute could occur soon.

Even the majority of Americans who support capital punishment should be concerned by the possibility that it can be imposed on teen-agers. Dozens are on death rows now. Of the 39 states that allow the death penalty, however, 10 will not impose it on persons under 18. Some of the impetus for these exemptions stems from the efforts of the American Bar Association, which has taken no position on the general question of capital punishment but has strongly opposed its imposition on those whose crimes were committed before they were 18. Support has also come from religious groups and others -- such as the author of the Maryland bill, Sen. Decatur Trotter -- who opposes the penalty under all circumstances.

It is disheartening that Maryland has chosen not to join the company of enlightened states that ban the execution of minors. One of the reasons advanced for opposing Sen. Trotter's bill is especially inexplicable -- even bizarre. Carving out an exemption, says Sen. Frederick Malkus of Cambridge, would be "an incentive to people who commit murder. (They) will say, 'If I'm going to do it, I've got to do it soon because I might be over-age.'

We've heard no evidence that in the 10 states where executions under 18 are prohibited there is a rush to murder before the fatal birthday. Concern about draft registration, maybe. Pride n being able to buy a beer or vote, perhaps. But anxiety about completing all your planned murders before the possible penalty increases? Sen. Malkus has got to be kidding.

Sen. Trotter intends to reintroduce his bill in the next session of the legislature. He is right to work against the death penalty for young people. But he needs to reconsider the provision of his bill that would require a mandatory life term without possibility of parole for a teen-ager who, under the existing law, would otherwise receive the death penalty. Mandatory life, with no parole, for a teen- ager? It is an alternative to the death sentence, but a very harsh and narrow alternative. The Maryland Senate needs to continue its review.