You can add additional children to your tax free childcare account. You get the same amount for each (I think it's up to £2000/year?, so it would be £4000 for 2 children). You have to pay money into each of their accounts then you can transfer it to their childcare setting. It's a bit annoying having to do it separately but if you set up regular payments then you don't need to worry about it every month
I'm under the impression it's 20% off each bill as each child will have their own code x