I gave notice to one family due to them making it very clear they used my services instead of a nursery as they knew they wouldn't get away with sending sick children etc. there amongst other things. I was pg with ds2 and decided that enough was enough and they couldn't do this any more. I did, however, put in my emailed letter than it was due to a change in circumstances (fit in well with my pg!). Before you ask about the letter, they were away at the time plus I hardly saw the parents as the au pair did drop offs/pick ups. All ended ok with no problems to follow. I was so lucky to be able to see these children after as the new childminder went to one of the same groups (I was gutted about the notice as adored the children, one started at 6 months old, the other at 5 weeks old). I still see them on occasion, over 4 years on :)
Another I gave notice to in the settling in period (saying I would not be caring for them the next day) due to finding out they knew very well how sick their child had been when they came for their first setting in (such a bad sick bug that 13 people linked to my setting got violently ill, children and parents). I'd lost hundreds of pounds but when they begged me to change my mind and offered to pay the money back, as hard as it was I stood my ground. This person sent some nasty emails a couple of times but that's it really.
The last one was a harder one. When I worked with dh, we had 6 children ages 2 and under but when dh stopped working with me (several reasons), I obviously couldn't keep them all so worked out most of them with reducing days/variations so I had 4 each day. This one little boy was so lovely but incredibly hyper and so was my ds2. They were both climbers and thought nothing of climbing up to the tops of the sofa and then trying to stand up and jump off. I had them under control but with the poor other 2 mindees being left more to it. So it was like they were being punished for being good. The problem was neither boy was so bad on their own but put them together and it was horrendous. So I sat his mum down, told her I adored her child but explained the problem with both of them making each other worse and although we both cried, she understood as she found him hard on her own. I told her that I had to think of health and safety and the fact that Ofsted had approved the variation and should I just carry on, I was worried something could happen and that Ofsted wouldn't like it if they knew I was putting people at risk with these 4 children in my care. We remained friends, although haven't seen as much of each other more recently due to work etc.
I find all this really hard so I'm pleased that I've not had much trouble as a result. I only give notice for very good reasons.