Because that's not what singular means. If you want to say that their beds are made for one person, you should say that they are singles, or single beds. If you want to say that the children sleep alone, you should say that they sleep alone, or one to each bed. Or separately, as in your first example.
Singular means 'the only one of its kind', when it doesn't refer to a singular noun.
'She had a singular ability to create chaos and discord.'