This sentence has no "a" and no "of". One of those swedish sentences that omits unnecesary words.
We could say in English "children need much sleep". In English it sounds...awkward, maybe a little stuffy or old fashioned, but it would be grammatically correct.
Please don't get me wrong: "Children need much sleep" is by no means a good sentence in English. It just doesn't sound right. But it's not incorrect.
If you negate that sentence on the other hand -- "Children do not need much sleep" -- it's fine.
Why is it correct to say "gor i somnen" but not put the en on somn here?