I can empathize with you because it got like that to a point with my younger cousins (who were at that time probably about 1 and 4). T's told me (and now I tell/agree with you that) they are not my (or your) responsibility - but I would feel like without me, they're parents wouldn't have childcare, may lose their job, they'd be hungry/homeless - and it'd be my own selfish fault.

What I had to do was stop being available. I would grab my laptop early that morning, and go to Barnes & Noble or Starbucks for the day. It felt a little weird at first, but it became so relaxing and empowering and much easier to say no. My mom, like yours, mentioned the overuse of me to them but it didn't change much. So when I stopped being available, she had to fill in and it didn't take but a week before she became sick of it and told them they needed to get their 'stuff' together. It didn't completely change the situation but we have them less often and with my permission. They are over three to four days a week now vs. six/seven; they were even coming over and staying the whole day on their days off of work. The kids were so used to coming to me for things that they would ask me even if their parents were right next to me (now I deflect those needs to the parents).

Not trying to hijack your thread, I just don't want you to feel you are alone in your struggle. I'm sorry to hear that you have to go through this, but i've honestly found the only way to put an end to it is to prove that you are unavailable. And, if all else fails, I would definitely do the no door answer followed by dropping the child off at work with the parent. The parent doesn't regard your time, feelings or business and so it's time to show them what it feels like. Sometimes you just have to say enough is enough. You are worth more than the treatment you are allowing. It will be hard to be the 'bad guy' but remember - you've tried the nice/decent approach. This isn't something that just started and you're overreacting. This is your health, happiness, time, energy and ultimately your choice. Even if you had no reason for not wanting to watch them - that is your prerogative. As you said, they are not your kids. No one can be a better advocate for you than you.