Questions & Answers about Дети шепчутся в комнате, когда мама спит.
The verb here is шептаться, not шептать.
- шептать = to whisper something
- шептаться = to whisper among themselves / to whisper to each other
So -ся often gives the verb a reflexive or reciprocal sense. In this sentence, Дети шепчутся means the children are whispering to each other, not just whispering words in general.
Шепчутся is the correct 3rd person plural present form of шептаться.
Its present-tense forms are:
- я шепчусь
- ты шепчешься
- он/она шепчется
- мы шепчемся
- вы шепчетесь
- они шепчутся
The stem changes from шепт- to шепч- in these forms. That kind of consonant change is normal in Russian conjugation and just has to be learned as part of the verb.