Choice of childcare day

just do what you want, offer what suits you in clearly defined terms, eg
DS/DDIL, i could look after your child on wednesdays if that would suit you, let me know.
and don't be guilted into doing anything you don't want to do.
to look after anyone's children, regularly, without pay, is doing them a great favour.
hope they'll repay it if you ever need looking after.

very very unreasonable.

What’s VVU please ?

I can’t see why on earth it wouldn’t be fine for you to say which day you’re willing to have her! Anyone objecting would IMO be VVU.

Yes of course it is Notjustaprettyface. The parents will ultimately decide if it 'works' for them but you're entitled to manage your time as you see fit.

Thank you and yes I am unpaid !
Why do you think it might cause family discord ?

Yes it is- especially if you're unpaid. I think it would be lovely to have the cousins together but I would play this very carefully as you don't want to provoke family discord.
It is really the parents decision but I hope they choose this option. You sound like a lovely gran!