ZAGREB, Yugoslavia (AP) _ As their young sons play a game of soldiers, parents in the bomb shelter of 53 Gundulic St. sip homemade plum brandy and try to laugh off the real fighting raging in their secessionist republic.

''The kids want to play (Croatian) National Guard against (Serbian) Chetniks,'' said Sanja Plavljanic-Sirola, balancing her toddler Teo on her knee as she sat on an upturned orange crate in the apartment house's dimly lit cellar.

''But the problem is, none of them want to be a Chetnik, so it's always the smallest ones who end up being the 'baddies,''' said the young mother.

Croatia's 4.75 million people include 600,000 ethnic Serbs, many of whom do not want to break away from Yugoslavia. Ethnic Serb rebels have taken up arms since the June 25 independence declaration and are backed by the Serb-led federal army.

Although air raid sirens have shrieked every day, many of Zagreb's 1 million residents refused to believe federal warplanes would launch a serious strike here. Now, with fighting hitting home in towns and cities throughout Croatia, that view is changing.

As sirens droned Saturday, Jeka Rupic took shelter in a shopping passage under Zagreb railroad station.

''It's been serious all along, but now you can't count on anything,'' said the 34-year-old streetcar driver, who had been en route to work.

''The generals feel they're losing ... and that makes them dangerous,'' said Mrs. Rupic, an ethnic Serb married to a Croat.

She said she had ''absolutely no problems'' with her Croatian colleagues at work. More of a problem was driving her streetcar at night without lights through Zagreb's blacked-out streets.

In the Gundulic Street cellar, neighbors huddled, sharing a bottle of homemade plum brandy.

''It's our duty-free shop,'' joked one resident, Ana Ruzicka, pointing to a small collection of bottles atop another crate. ''The other night we had to sleep down here, so we toasted with good wine in crystal glasses.''

But beneath the bravado, there is fear.

Tanja Draskovic, visiting friends in Gundulic Street when the alarms sounded, said she held out little hope the international community could stop the violence that has killed about 500 people since the republic declared independence.

Mrs. Ruzicka said all but two of the house's nine families were using the cellar.

A neighbor, a Montenegrin, whose republic is allied with Serbia, had never used the shelter, and other residents grumbled she left her apartment's lights on during air raid black-outs.

''We think she's working for the other side,'' muttered one resident from the back of the shelter.

The six boys in the shelter, ages ranging from 4 to 8, raced around the room, imitating the sound of airplanes. They marched to the tune of the Croatian Guards' march.

One parent shouted to the children to turn off the radio to save batteries. During the hours underground, the radios are often the only link between the cellars and the outside world.