Absolutely not a quango. The Commissioner has done some very thorough work and is often hugely critical of Government policy like the recent report on disability. The link you have is to the child's version. See this
They have also done some very good work on exclusions.
Also, the appointment of a Commissioner was intended to honour commitments under the UN Convention on the Rights of the Child.
However I suspect their budget is crap and I know their mandate is limited. Having said that those Commissioners appointed thus far have done their best to shout as loudly as they can.