Brief Answer:
in my opinion it would be best to be upfront and honest

Detailed Answer:
Hello ma'am and welcome.

Thank you for writing to us.

I appreciate you seeking my advice, you seem be a warm-hearted and emotive person who shall surely be blessed by the Lords bounty for your kind and selfless deeds. I understand how disappointed and thwarted you must be feeling after all the good you've done, most people these days don't appreciate kindness and often tend to take advantage of the good ones as is in your case.

Coming to your question, these issues can be very tricky because all you want to do is help and be appreciated, and instead you get bogged down for that one genuinely busy instance when you had to take an examination. I want you to be prepared in case this woman contradicts you while you are trying to vent your feelings out to her seeing how unappreciative she has been in the past. There can be no beating around the bush, you have to just give it to her straight, be upright and try to explain your plight to her. You need to stop being the scapegoat here.

On the other hand if she understands what your pointing out to her and takes it in good stride then there's nothing like it. I really hope she does, for you deserve nothing but kindness and warmth.

But it is highly unlikely that she will which is why you could come up with an excuse for that particular night, stating something like:

-You have to study all night as you have an examination the other day
-You are not going to be at home or are going to return late (due to so and so reason)
-You are not keeping well
and so on.

But, I would also like to add that this will only help dodge the soon-approaching date. What if she approaches you to babysit her kids again in the future? Will you be able to come up with excuses each time? It is certainly going to be difficult which is why I think it would be best to be upfront with her and tell her how you felt the last time and that you are not ready to be used or to be unappreciated anymore. Let her know that you love her kids, but are not ready to go through what she put you through the last time.

Unfortunately disunity is not the issue here, its about peace of mind ma'am. If you choose the option wherein you have to keep making excuses then this is going to develop into a vicious circle, and she will soon realise that you are dodging her, and might lash out on you. You have to take a stand and let her know how you feel and that you cannot be bullied around any further.

I hope and pray all goes well, and also hope you do not have to live a life worrying about her and making excuses to her. Please also feel free to write to me anytime, I am always here to help.

Best wishes.

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