Ideally, none of the children should miss out/get less because they have lots of siblings, but realistically, that's going to be their whole life in a very big family, in so many different ways. They didn't choose to have so many siblings, but their parents did choose to have so many - and for each of those children to get less/a smaller share of just about everything.
It's interesting how they expect you to be able to keep affording the same amount for an unlimited number of children, yet they 'rely' on it (which could be open to some interpretation), suggesting that they themselves can't keep affording it.
In one way, it's unfair if each child in the massive family gets less, but in another, it also means that - depending on their ages - they can benefit from sharing whatever their siblings spend their money on; ergo a family with two children have only £40 of toys/games available in their house whilst a family of 8 will have £160-worth in theirs, as long as they don't spend it on consumables.
This will probably sound weird, but the only truly fair way I can think of is for you to give the same amount to the parents of the big family as to the parents of the much smaller families, then agree that the children will all get £20 each from you, BUT the parents add the rest on your behalf to make it up, so that each of their children still gets £20 'from Aunty Sue & Uncle Mike'. That way, nobody is penalised for circumstances beyond their control and the only people who have to pay out more are those who actively decided to have a very large family, knowing full well that it would end up costing them a shed load more.