Sussex Police are asking children as young as four to join a crackdown on firearms.
Letters were sent to all schools warning parents of new laws which will make it an offence to carry replicas, air weapons and BB guns in public.
Children, many at infants schools, took the letters home at the start of the summer holidays.
Anne Smith from Brighton, the mother of a six-year-old boy, said: "It made me laugh.
"Children in reception classes could barely read the letter let alone understand it.
"And does this mean kids can't play cowboys and indians in parks anymore?"
But Sussex Police are deadly serious.
They will not intervene if children are seen with water pistols or guns which are clearly toys but armed officers might be called in if weapons look real.
During half term earlier this year, police were called to numerous incidents of youngsters firing BB and air guns.
In Whitehawk, Brighton, a two-year-old girl and a 14-year-old boy were hit by youths taking pot shots.
Police firearms experts are being called out to BB gun incidents two or three times a week in Sussex and in the majority of cases, the firearms unit is deployed.
Between April 2002 and March 2003, Sussex Police attended 237 incidents where people were reported to be using handguns. Most turned out to be fake.
Assistant Chief Constable Geoff Williams, in his letter to parents and guardians, outlined the reasons behind the launch of the latest anti-gun campaign, Operation Holster.
He said: "The use of these weapons has increased over the past few years, particularly by young people and that's why I am writing to you - we need your help."
He said it is already an offence to carry loaded air weapons in public and the law was about to be toughened up, making carrying them without a good reason and using them in public illegal.
Mr Williams said: "Even our trained firearms officers cannot always tell them apart from the real thing.
"Anyone carrying or using one of these weapons in public may find themselves confronted by armed police officers. This is something we obviously do not want to happen as it increases the risks for all involved.
"If your child or children either have one or have access to one I would ask you speak to them and ensure they are aware of the law and the consequences of their misuse.
"The simple message is don't carry them on show in public or use them in public."
Chief Superintendent Paul Curtis, head of Brighton and Hove police, said: "We are not out to cause alarm or spoil children's fun but youngsters are getting hurt and we have to do something."
Operation Holster has received the backing of Dr Philip Dodgson, head of psychology services with Southdowns Health NHS Trust.
He said: "I support the move because people have been injured and killed as a result of the use of imitation firearms.
"We are not talking about water pistols or multi-coloured toy guns but the realistic-looking weapons."
He did not think the campaign stop children playing cowboys and indians: "A friend's children used sticks as pretend guns instead."
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