I would have to say cash all the way.. And a week's wage is generous if she works full time for you, but if it's part time, I would up the amount to perhaps 2 week's pay.. Not sure how often your cleaner comes, but I would definitely give her as much as you can afford, as often they are struggling the most.. Remember it's xmas, and it's always a challenging time of year for anyone whose earnings are mediocre in comparison to our own household finances.. (Cleaner's are often the worst off)
If you want to, perhaps a little pressie to go along with the cash is always appreciated (a warm winter scarf, or nice gloves..) .. Perhaps a nice homemade card from the children to add that personal touch..
Bottom line, give generously, but obviously, limit to what you can also afford. Your nanny and cleaner will appreciate it, and it will only strengthen your relationship..