Yes it is fine, it just sounds odd because it is very short and alone.

Imagine if the student had made it a longer sentence about a whole family day out, it would sound fine.

The dad is fishing, the mum is sleeping in the long grass and their kids are playing a ball game.

I agree it does sound odd as presented, and it took me a few moments to realise it was a correct sentence.

Yes it is fine, it just sounds odd because it is very short and isolated.

Imagine if the student had made it a longer sentence about a whole family day out, it would sound fine.

The dad is fishing, the mum is sleeping in the long grass and their kids are playing a ball game.

I agree it does sound odd as presented, and it took me a few moments to realise it was a correct sentence.