More importantly: Do you need a nanny? No? How about a babysitter? Best friend?
This is an incredibly considerate question! I obviously can't weigh in on your friend or friendship - it seems reasonable enough that you would be allowed to clarify a second time, but a more sensitive person may take your concern as a suggestion that she doesn't treat her nanny well.
I also don't know this nanny, but you can always think about these relationships like any other long-term, intimate relationship: Rewarding, completely worth it, but not easy. In all likelihood, both the nanny and your friend have come to take each other for granted on occasion. Although there is an implicit balance there (who doesn't take the people their closest to for granted at times?), the power-dynamic of the employer-nanny relationship is one in which the nanny will never feel as empowered to speak up or defend herself as the employer. This can lead to greater or faster resentment on the nanny's end despite also appreciating a fundamentally healthy and mutually-beneficial relationship.
SO. Those are the broad-strokes that you can reasonably assume for most employers and nannies. I'm assuming that if your friend were abusing her nanny she a) would not have had the same one for 8 years, and b) would not have a close friend this concerned with the welfare of others. That said, I have to be honest, spending the summer at a beach house with my employers and not being able to leave (really? at all? ever?) does not sound like a great time to me. If what made that worth it were having a significant amount of downtime (Maybe she's a student? A reader? An introvert?), then you're right to think you should acknowledge the times when you're tripling the responsibilities she expects with only one toddler. Regardless, your children are not a part of her work arrangement, and no matter how little concrete effort it seems to add, I think anyone would appreciate the courtesy of being asked!
It's only a week, it doesn't sound like you intend to leave the girls with her often, so my suggestion would be that you not risk making a big issue of it, but that, before your trip, you plan a way to show your appreciation to the nanny. Tally up an estimate of the number of hours you plan to use her (do not distinguish between when the children are or are not sleeping - it's all time that she cannot use for herself) and come up with something you think is commensurate with that value.
Is she the type to pamper herself? Does she eat out? Anywhere in her hometown where you could get her a gift card or reservation for when she's back from the summer house? Everyone loves a gift certificate - I don't know where you live, but Target is generally a safe bet or, since the summer is only half over and you say she can't go anywhere, a gift card to Amazon.com (which you can pick up at most drug or grocery stores) would allow her to treat herself to something without having to request the family car or time away.
Regardless - be sure to thank her profusely for her help while you are there and, if you plan to leave your children with her when they're up and busy and wondering why their siblings are off on a special boat ride, then also be sure to provide her with some activities and resources to make her time with hem as smooth as possible. The best way to do this is by preparing your children in advance, getting them excited about THEIR special time, which is easily accomplished by engaging them in the planning of those activities. That way you will hopefully minimize any potential meltdowns (but don't panic if they can't be avoided - it happens).
Then pick up a thank you card & throw a few crayons in your bag. On your last night, write a nice note in the card and have your children color/sign it too. Add the gift card and leave the envelope for her before you head out. Hopefully that will be subtle enough not to ruffle your friend's feathers but, taken together, a more-than-appreciated gesture for the nanny.
Thanks for such a generous question and enjoy your vacation!