"I don’t only donate my time or money to specific individuals depending on how they’ve treated me."
No? But you donate based on how worthy you think they are - because a donation you give one charity is a donation less for another. So how do you decide who's worthy of your money or not?
"If everyone only donated / volunteered for things that directly affects them then most of the world would get no help at all."
Yet you are obviously expecting the OP to donate to someone who directly affected her, negatively. That's the only reason right that OP should choose to save this awful woman's child rather than a stranger's child - otherwise why does this child deserve money more than another?
"If you wouldn’t donate even £1 to potentially save someone’s life just because you have a grudge, even though you could easily afford it, then that does not make you a nice person."
But that £1 you are giving to save this life is £1 less that someone else will get, correct? Because we all have limited resources to help others - so why is saving the child of an awful woman better than saving the child of someone who's done you no wrong?
"I hope if my child was poorly the people that could afford it would donate a couple of pound to potentially save her life."
Hopefully you wouldn't be the sort of person who destroys people's livelihoods and careers - and then expects them to bail your family out. Helping out bad people, even if it is supporting their families, is enabling the bad behaviour. A bit like you supporting the fund raiser for a mob boss's sick child or your abuser - the child may be innocent, but they aren't separate to the parent - and indirectly you are sending the message that it's ok to mistreat people. It's not brave, generous or being the bigger person - it's ensuring they keep behaving badly because they know they can get away with it. And at some point in the future OP, will look back and feel upset that she was manipulated by this person yet again.