Err... If Christmas is only one statutory holiday and she got four days, I'd be pretty stoked by that. (If she has no concept of money, though, maybe she doesn't realize your gift = X number of presents she might have wanted.) Maybe it's a cultural thing (some cultures cash gift is normal, others may value large consumer products wrapped up with a bow?) If she showed up with a big bag of toys for your twins maybe she finds more meaning in a physical gift.
Practical consideration: how good is your nanny and how competitive is the local market for nannies? I don't have a nanny but hear that around here, poaching is a real worry. If nannies are in-demand, sometimes you just have to pay and gift visibly better than the competition (yeah, it contributes to wage/expectation inflation, but is probably worth it to avoid her jumping ship.) I know it's a cut-throat way of looking at it but I would think of it as part of her overall pay and what it takes to attract and keep a desired employee.
She is absolutely fabulous with my babies and helping around the house but she isn't super reliable (which obviously is a huge part of the job). She routinely leaves me in a tight spot 1-2X per month by calling in sick with a number of different ailments (throwing up, headaches, broken arms twice in the last 6 months). She called in sick on Tuesday of last week and then got in a wreck on Thursday that was her fault. Last week was kind of a disaster for me work wise...good thing my work is VERY flexible.
... Probably not your concern, but: if she's flaky and injured, is she in an ok home situation?
Perspective on the gift: I send my daughter to a group daycare, which costs 125$ per week before government subsidies (we wind up paying about 40% of that out-of-pocket. Yes, I am aware that we are fortunate). The daycare is closed for the 2 weeks around Christmas, so we don't pay but have to take care of our daughter (yay vacation!). We gave the lady who runs the daycare a card with 60$ (logic: our mail lady gets 30$, and she only delivers our MAIL, she doesn't take care of our kid every day! 2-3 days of pay seemed reasonable, if slightly under-generous, but finances being what they are, that's what we settled on). She was extremely surprised, and protested that it was OVERLY generous. I asked my SIL what she did: her family produces maple syrup, so she gave everyone 3-4 cans (value: 20-25$CAD) as presents. My colleagues, when polled, either gave in the 50$ range, didn't give anything, or gave an 'easily re-giftable' present (wine, food, hand soap set, etc).
4 days of vacation + 100$? Lady. I have a corporate job that pays really well, and I'd personally be stoked over that as a Christmas gift from my employer!