Questions & Answers about Reszta dzieci śpi w pokoju.
Because reszta is followed by the genitive case, and the genitive plural of dzieci is also dzieci.
So:
- reszta = the rest / the remainder
- dzieci = of the children
This is a common pattern in Polish:
- reszta ludzi = the rest of the people
- reszta czasu = the rest of the time
- reszta dnia = the rest of the day
So reszta dzieci literally means the rest of the children.
Because the grammatical subject is reszta, and reszta is singular.
So Polish agrees the verb with reszta, not with dzieci:
- Reszta dzieci śpi. = The rest of the children is sleeping / are sleeping.
This can feel strange to an English speaker, because English often focuses more on the people involved than on the grammatical head of the phrase.
In standard Polish, singular agreement is the safest choice here:
- reszta dzieci śpi
You may sometimes hear plural agreement in less careful or more informal speech, but śpi is the standard form to learn.