It just sad to think ds might not have nappies!
And that kind of emotional manipulation is exactly what she's relying on. That people would rather give her money (because you're not lending it if you never get it back) "for the children" than feel guilty for calling her bluff and potentially the kids genuinely going without, but it is not your responsibility.
It would be one thing if it was a one off or a rare occurance - I have had friends need to borrow a tenner at the end of the month for the electric, but because they do it so rarely and pay me back in kind, whether it's babysitting my DD or feeding the cats when I'm away, that I am more than happy to write off the occasional "loan" and don't ever expect repayment.
However, this woman is getting £30/40 a month almost every month from you because she is spinning you a line ... and goodness knows how many others as well. £40 a month could pay for DD and I to go out for dinner and to see a film as a treat; allow for a few treats in the shopping; go towards DDs new school shoes etc. If you can afford to give away that money, then fine, but if it's leaving you short then why does her family come before your own because of her failure to budget properly?
In future I'd just respond to any requests with "I'm sorry but you still haven't repaid the last £20 I lent you a couple of weeks ago. I cannot afford to give you any more money because it's leaving me short."
I would even be tempted to send her a message - not in response to one of her begging texts/calls but at a neutral time, so she can't see it is a direct pushback to that particular request - to ask her not to call/text for money any more.
"Dear Freeloader,
This is not an easy message to write but unfortunately it has now become necessary. Please do not ask me for any more money as I simply cannot afford to continue this way. You regularly call me to lend money but I rarely, if ever, see any of that money back and I am starting to feel like my generosity is being abused.
In the last month you have asked me for money three times, and I have given you £20 each time and you're asking me for another £20 despite that fact you've paid back none of the other "loans" or even offered. I simply cannot afford to give away £80 a month on a regular basis and so from now on I won't be lending you any more money. "