"Be quiet. They are sleeping." would be a correct alternative. Posted on Nov. 19, 2017.
Could "tamen shuijiao le" be used instead of "tamen zai shuijiao" to be achieve the same meaning?
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.