The splurge guns did not actually fire the "splurge". Director Alan Parker first tried wax balls filled with cream but these hurt when fired, so in the end the splurge guns actually fired ping pong balls, which the actors fired at nothing and what we see on-screen is clever editing between this and shots of actors being hit by handfuls of cream thrown at them by others.
When looking for Fat Sam, director Alan Parker went to a Brooklyn classroom and asked who was the naughtiest boy in class; all the class replied John Cassisi, who subsequently got the part.
Florrie Dugger was at first only given a minor part - until the original actress meant to play Blousey underwent a growth spurt and became taller than Bugsy (Scott Baio), and Dugger was given the role of Blousey.
Although it performed well in England and Japan, Paramount only gave "Bugsy Malone" a limited release in US theaters, usually dumping it onto second-feature screens partnered with a late-'76 re-release of The Bad News Bears.