"安静！他们在睡觉。"

Translation: Quiet! They are sleeping.

[OKAMOTO_Yusuke]

"Be quiet. They are sleeping." would be a correct alternative. Posted on Nov. 19, 2017.

[Susie478963]

Could "tamen shuijiao le" be used instead of "tamen zai shuijiao" to be achieve the same meaning?

[KX3.]

Interestingly， the (or a) correct answer is 他们睡着了 i.e. "They have fallen asleep" or "They are asleep" where 着 is pronounced zhao2.