их было не слышно и не видно ling.

[QueenKong]

Подскажите, пожалуйста, есть ли устойчивое выражение для перевода данной фразы:

Дети вели себя очень дисциплинировано, их было не слышно и не видно.

Спасибо!

[Translucid Mushroom]

as quiet as a mouse/grave

[Аристарх]

meek and mild
meek as lambs

[kinsman]

имхо:
made themselves inconspicuous/were keeping a low profile/as if were removed out of sight, out of mind

[Translucid Mushroom]

as quiet as a mouse

as quiet as THE grave

Perdoname (:

[Althea]

Children were neither seen nor heard.

Good old rule of "proper English upbringing" - children should be neither seen nor heard.