I have had similar internal debates over the last few years.
We have a nanny. She is on maternity leave now, but she has been with us for 4 years. I think she is great (not perfect, but then neither am I), and she works 3 days a week (long days) as do I.
I have had a salary increase each of the years that she has been with us, at around 2 or 3 %, and an annual discretionary bonus (of in the region of 700-900).
I have always passed the percentage increase on to her, and paid her a proportion of the annual bonus. The proportion has been between a third and half. She came to us earning market rate (and for her, this was more than we paid our previous nanny, as our current nanny has more experienced) and so I think she now earns above market rate on an hourly basis. I paid the bonus because she was super reliable, took very few sickness days, was very punctual, and I felt that she deserved a bonus. I did make it clear that she was getting a bonus for those reasons. A bonus is completely discretionary, and so you shouldn't feel that one is compulsory.
I would give whatever you are comfortable about giving, and what you can afford to give. And make it clear why you are giving it. What is the market like for nannies where you live, and are you paying market rate/below/above market rate already? What does your contract say?