Help me to not BU regarding Secret Santa presents for siblings

[Dungandbother]

I have gone in circles and nobody has been unreasonable YET so this is a backwards AIBU thread

What is the fairest way for my siblings and I have a secret santa pot which is fair to everyone in our different circumstances? Help me...

Siblings
A - couple, two children
B - single mum, two children
C - couple, two children
D - couple no children
E - couple, one child

5 families
9 adults
7 children

We've agreed on a secret santa type pot so everyone gets one present of equal value. And less 'stuff' for the children.
Everyone agrees to spend more on the children than the adults.

We all typically spend £15-20 on each child and we've done all sorts of deals with adults - some agree not to buy each other to save money whilst others are happy to spend regardless.

It is hurting my brain. Should each family put money in the pot, or each adult?
How do we prevent unfairness to the childless couple? Or the single mum?

So is the idea that everyone gets one present each, including the children, but the children's present might be worth more?

And then I assume that only the adults are contributing!

So why not fix up how much it will cost and then divide that by the number of adults. An equation might help. E.g.

x = Amount spent on each adult
y = Amount spent on each child

We know there are 9 adults and 7 children so the amount each adult would contribute would be:

(9x + 7y) / 9

So if for example you decide on £20 per adult and £30 per child then each adult contributes:

((9 x £20) + (7 x £30)) / 9
(£180 + £210) / 9
£390 / 9 = £43.33 (apprx)

You could then play with x an y until you get a sum that satisfies everyone