According to The Sun, ITV bosses issued the demand in response to the controversy caused last year when singer Hollie Steel and other child acts burst into tears live on air.
An ITV source said: "Contestants' welfare is our number one priority, especially when it comes to kids. There's no way we want a repeat of last year when Hollie Steel broke down.
"Most of the adults can take the stress of being told they're no good - but the kids can't. We've told the judges to go easy on the kids and make sure any criticism is balanced with positive comments."
It is thought that producers originally considered introducing an age limit to the contest this year, but later ruled out the idea after deeming the young acts 'too popular' to lose.
The insider explained: "The kids really lift the show as it's about showcasing extraordinary talent."
Britain's Got Talent returns on Saturday at 8pm on ITV1.