I get this from DD in particular, and I usually respond by laughing (bad mummy) because she looks so funny when she says it. I also say "oh well that's a shame because I love you very much, but you are still not getting x, y or z". Sometimes I also agree with her when she says things like "you're a bad mummy" and I reply "that's right I am the worst mummy in the world and I don't know how you stand it, but you are still not getting x, y or z" Honestly it doesn't hurt when she says it because I know she doesn't mean it and for me to agree really takes the sting out of it for me, and the impact out of it for her. She doesn't do it much anymore so it appears to be working anyway