Really? A nanny's holiday bonus isn't typically a week?
http://www.4nannytaxes.com/index.cfm/resources/news-and-updates/year-end-bonus/
the average year end bonus is one - two weeks of pay
http://www.care.com/child-care-holiday-tipping-a-holiday-bonus-guide-for-your-caregivers-p1017-q3901313.html
A full time nanny can usually expect a holiday bonus of one week's salary. Some will get more (or less), but one week is the norm.
http://www.nannypro.com/content/pages/nannybonus
Cash bonuses: If this is the route you can afford to take this year, the average year-end bonus is two weeks of pay. For newer nannies who have just recently been hired, many families calculate a day's pay per each month the nanny has been with them to figure the total amount of the bonus
I know I've read other articles about this, but I can't hunt them all down.
Do people increase salaries on here? Sometimes they do, absolutely. I highly doubt that there are many nannies (if any) out there that make $20/hour or more for one child.
But in the terms of a holiday bonus, 1 week is standard.