If you would employ her again. I have this in most of my references and perspective employers really like this, they also like the fact that I have gone back to 4 of my previous families for a week of proxy parenting/gone on holiday/helped out when current nanny left at the drop of a hat etc.
Also mention the reason she is leaving (if it is on good terms)
I guess a lot depends on if she is leaving on good terms or not. You cannot legally give a bad reference, you can give a basic reference, dates, ages of children and any good points but you cannot say she was a lousy time keeper, which, even as a nanny, I think is madness! You can go into more detail in a verbal reference, if I recall correctly.
If you really like her, had a good relationship and she is leaving because of a change of circumstance, then please gush about her in her reference! Write all the good things about her, time keeping, cooking, arty, organisational skills, if she had time off sick (in my 25 years of nannying, my references say that I have had the grand total of 9 days off sick and that I sometimes turned up when I really should have been in bed) All of my references are two A4 sides and a most of them say, she is and always will be a part of our family (which I think some of them regret saying as I am still very much in contact with all of my (22) previous charges, even the 25 year olds! In fact I am currently living rent free in a cottage in the grounds of a family that I left 10 years ago (I do the odd spot of babysitting (they are almost 14 so don't really need me) school pick ups and look after the dog/cats/chickens/house if they go away for the weekend.)