You've probably noticed that toy guns – mainly in the USA, but also sometimes elsewhere – have a bright orange tip at the end. In some states, that orange tip is illegal to remove.
In the US in the 1930s, toy guns were blamed for perceived increases in aggression in young people. In a good old-fashioned moral panic, toy guns were even burned in protest. Later, as the popularity of the toys dwindled, gun control advocates pushed for toy and replica weapons to be brightly colored in order to distinguish them from real weapons.
Though these attempts were unsuccessful at the time, in 1992 the Department of Commerce legislated that all toy guns should have "a blaze orange [...] or orange color brighter than that specified by the federal standard color number, solid plug permanently affixed to the muzzle end of the barrel as an integral part of the entire device and recessed no more than 6 millimeters [0.2 inches] from the muzzle end of the barrel".
An alternative was to have a blaze orange (or brighter) thick stripe on both sides of the toy's barrel, or for the gun to be brightly colored or see-through.
As always with American law, there are differences from state to state, with New York legislation specifying what colors toy and replica guns can be.
In most of the US, it is the manufacturers who need to worry about this. In California, however, you could find yourself at the barrel end of California Penal Code 20150 if you remove the orange markings from your toy gun. In that state, you can receive up to six months in prison and/or a $1,000 fine if you remove or otherwise alter the toy or imitation gun's markings. The entertainment industry is exempt from this, which is why guns in TV shows don't have bright orange tips.
However, prosecution for your child accidentally removing the orange tip from their gun is unlikely, no matter how often you report them to the feds.