LONDON — Western security forces have a new frontline in the battle against Islamist terrorism: the under-fives.

Britain has drawn up plans to force kindergarten teachers and registered child-minders to spy on pre-school children in a bid to halt the radicalization of a new generation. Two young children were also taken into protective police custody last week when their mother, who was questioned over terror-related offenses, returned to Britain on a flight from Turkey.

The apparent crackdown on pre-pubescent Muslims marks an extraordinary expansion in Britain’s already controversial counter-radicalization strategy.

Alarming images of young British children pictured with weapons in the so-called Islamic State prompted officials in London to say