Yep it all seems pretty non-sensical I would agree. You can get the child working tax credit bit (or whatever it is called) to pay for childminding fees, BUT the childminder has to be a registered childminder. i.e. someone who in effect does it for a living and has the necessary qualifications (yep all of the Government's fluff !). It is all the child protection garbage that this government has introduced by the bucket fuull over the last 10 years. Equally that person will have had to be enhanced CRB checked etc etc.
If you yourself have done all of that, then as far as HMG is concerned there is not the slightest problem with you childminding your family, nor anyone elses.
What tax credits will not do is to pay for childminding costs from someone who is not official registered. In fact when one applies for the credits for childminders, the official registration number of the establishment/person who is to provide the childminding has also to be quoted.
What alternatively can happen is that private arrangement between a childs parents and a friend (eg yourself) can be agreed, BUT providing there is no money etc changing hands the childminder DOES NOT have to be formally registered. This was the recent case of two woman pc's who each minders each other children when the other was on shift, where Ofsted attempt to argue they were illegal. Since HMG, has and looks to moving to allow private family arrangement, as I say providing no money/goods changes hand. All baloney if you ask me. AND of course in any of these privately agreed arrangement, you cannot get tax credits for them.
As you have discovered unless granny also childminds a none related children, which in itself requires granny to be formally registered as a childminders and could also demand she has some qualifications (you would need to check), there is no way that granny could formally charge and no way that mum could claim the childminding costs back/through tax credits.
It is is not a discrimination issue or indeed a human rights one, it is just the way it has has developed. You could claim it is a human rights issue, but HMG will counter that with their primary duty is to ensure child protection, and the only way they can do that is if all childminders are formally registered/meet certain standards, then of course in electing to pay/reimburse childminding fees as tax credits, they 'can' only be paid to registered childminders, as in their eyes there should be no other paid childminders in existence !
The fact that HMG still seems intent on making private childminding arrangements illegal if a fee/money's are exchanged, although most other parents think this is crazy, is something you are never going to get HMG to change it's mind over. Maybe if the colour of the Government changes this year, there might be changes....but who knows.
Hope this makes some sense