Interestingly in both books the magical nannies have had to come to help out the families because the badly-behaved children have driven away the former nannies. Mrs Brown fails to see that her children do anything wrong and has absolutely no authority over them; Mrs Banks in the first book has only recently had the twins and is rarely mentioned. The fathers seem to have no involvement with the children at all.
With nannies being less and less common for middle class families, there remains a problem of obtaining suitable childcare where one parent staying at home is an impossible dream. Who, then, steps in to look after the children? In Granny Nothing by Catherine McPhail there is Sue, the mean, reality TV obsessed nanny of the McAllister children (Stephanie, Ewen and Baby Thomas), whose parents are too busy to see that they are miserable. Then in the middle of a dark and stormy night Granny Nothing arrives, the mysterious mother of narrator Stephanie's father.
Mr McAllister is ashamed of his eccentric, fat, untidy mother, and initially only Thomas sees the good in her. However, as she helps the children deal with such threats as scary dogs and bullies, she wins over Stephanie, Ewen and their school friends, and ultimately is the conduit for Nanny Sue's downfall. While perhaps not magical, she certainly has special powers and is incredibly strong. According to Strident Publishing, the book was reissued due to popular demand from teachers, librarians and children, and I can quite see why: I read it on a train journey, and it is lucky my train could go no further without going into the sea, as I was enjoying it so much I would certainly have missed my stop!
With the exception of The Incorrigibles, I would suggest that these books could be read with great pleasure to 6 years +, read alone from 7-8 years, depending on the child's confidence. I would read The Incorrigibles to 8 year olds, but I think the humour and sophistication would be best appreciated by 9 years + (and stolen by parents and older siblings!)