Ok for all the reimbursement comments - I didn't think this was possible as I read at many places that the vouchers are non-reimbursable.. but yes of course my first attempt will be the voucher company / my company.
I understand the arguments behind people saying it is tax fraud, however I don't think it is so black and white. It does not seem like the fraud if the system allows you to do it effortlessly (i.e. back to what I was wondering originally - is it actually possible to pay for someone else's kid with the vouchers?), as otherwise the system would surely try to prevent it. E.g. consider child benefit - you get cash for your kid. There is no control what you do with this cash, you can spend it on whatever you want, and it is not fraud, despite it being 'for your kid'. Or cycle-to-work schemes - you can't just take cash but you need to buy a bike with it as otherwise it won't work, i.e. the system prevents misusing that money for something else.
Also, the whole point of the voucher scheme is to help me a bit financially to raise my kid. What's the difference whether I do it by using the vouchers, or I do it by selling vouchers and using the cash to help me raise the kid? (obviously, the cash value will be smaller than the voucher value as otherwise the other person has no incentive to buy it, so it's not like I'm finding a loophole to artificially boost my benefit, I am loosing money)
Yes I know it is on voluntary basis, but that does not make it fair - if I am not able to take my money back (of course adjusting for the tax breaks) then I would say voucher companies are making fools out of us all. Imagine you're paying into your pension for years, decide to move abroad, and you're told that the money is locked and cannot be used unless you come back to live in UK (of course, this is not how pensions work as - it would be crazy!). It's voluntary, I could have chosen not to pay into my pension, but that wouldn't make it right. Also, in many other countries, like France, Germany, Italy, .. you simply get a tax break for having a kid, and you can spend this however you want to, without all this voucher nonsense of locking away your money.
Anyway, sorry for the rant, thanks for all the responses :)