Buy a small pack diaper and give them...
Why will these things be an isssue anyways?
There are other ways to show your neighbour love apart from giving money.
I understand how you feel though,I make it a point to visit mothers and their new borns,so i get you.
If you can't afford that small sized diaper,please ask her if she needs help...Maybe a dish or two, depending on how close you are,Just something.