OP, just as a perspective from a family in this situation: we have a nannyshare because we feel it is the best child care situation for our child (though not to keep up with our wealthy friends--would that we had some of those to keep up with!

As a result, we pay a lot in child care--more than our mortgage (and that's for a share!) and a large chunk of our monthly income. We definitely do scrimp and save to make it work. But it's a decision that we made as a family, and it has an end point, when DC will go to preschool and our costs will drop back down again. In the meantime, yes, things are tight, but no, we have NEVER missed a paycheck for our nanny, and we don't skimp on benefits or bonuses, either (because we considered the costs of all of that when making a decision to go with a nanny). I would never expect our nanny to offer to reduce her hours or her rate to make it easier, even if she does observe that things are much tighter for us than for the other family in the share (as I'm sure she sometimes does). Much like I would never expect my employer to offer to up my hours or give me a raise after observing that child care costs a lot (not that I would turn that down!)
I would really encourage you not to worry too much about it, provided you are being paid on time and treated well as an employee. I'm quite sure your charge's family has thought through the implications of choosing a nanny versus daycare and they may well have their own reasons for doing so. (If you are really concerned, though, you could always broach a conversation about when they plan to transition to preschool. We have always been very upfront about when we planned to do this--both kids will start at the same time--but if you don't know if/when this will happen and the charge is getting to be preschool-aged, it could be worth asking.)