I love the way she describes it "just watching" them.. to make it sound so easy. You just have to look at them. Not babysitting or childminding.. which actually describes the task. I note she seems to just use the word just a lot to make it sound like nothing.
When she grinds you down to concede that you might possibly be able to watch them for a bit occasionally as an emergency, she will then announce as close to the time as possible that its much longer hours than she led you to believe and for at least several days... because you live to serve her, in her mind.
And of course because its happening the next day and she has no time to look for anyone else she will plead and will make it sound like YOU are personally letting her down, led her to believe you'd help and stopping her return to work and blah blah blah. So if you thought it was hard to say no at the outset - just wait until she really gets going.
At no point will she mention paying you because of course.. its just a small favour between friends.. but she will never look for other childcare when she has a free and biddable neighbour who has agreed to do it. And any favour from then on (and there will be more and more favours required) will ALWAYS be a last minute emergency, because then you will be under more pressure not to say no.
But you are not her mum or her auntie and you don't owe her.
"Hi neighbour, unfortunately I cannot just watch your DC whilst you are doing evening shifts. That requires proper paid childcare, from someone who is insured and has chosen to regularly look after young children. I wish you all the best but I cannot help you. I already have a full time job and I don't want to take on a second."
She will be offended, but if you helped her you would be living with the constant threat of offending her whenever you try to not do as she asks.. because job, because kids, because its an emergency. If she is offended and refuses to speak to you because you said no.. so blooming what? What can she top that with. Note that there will be no gratitude or acknowledgement for favours already done.
Unfortunately, you have to put up with this instance of displeasure on her part towards you ... to avoid a very long period of being her unpaid servant. If she knows its no good asking you, she will move on to the next one. Also.. ask yourself why you find it so hard to say no.. I bet you've were brought up to never say no etc. and were punished by people being mean to you when you did. But your life and your needs are just as important as hers. If she wants to be offended - let her.