Our nanny has gone above and beyond for us since she started in July and we'd like to give her a Christmas bonus. I want her to know how much we appreciate her and of course we'll give her a card too, but I know she's saving up so would rather give money than a present.
Is there a generally accepted rate for a Christmas bonus? I really have no idea! 
In case it matters, she's currently working three days a week and we're paying her £20k gross p.a. pro rata with all school holidays off.