Guaranteed hours are standard in nanny jobs, OP. Just as with any other form of childcare, you pay regardless of whether your child attends or not. You do have to educate new nanny employers about this, as you can see, because many would like to take advantage of your mutual ignorance by convincing you that their lack of policy is standard, but it isn't. Talk to your employers, use daycare as an example of something they'd have to pay for even if they pulled their son out for a week of vacationing, and make the offer to take care of things around the house while they're away.
An alternative is banking hours, but I never recommend that.
This poster is wrong. Guaranteed hours are a negotiated perk. An important and fair one, if you ask me, but not a standard by a long shot. It isn't the same as a day care situation at all, but that doesn't matter. What does matter is that your contract should have spelled this out so you wouldn't be in the position of having to bring up something like this later, when they may not be willing to alter the contract.
I do agree with the PP that you shouldn't bank hours. This could easily turn into a situation that would be inconvenient and stressful for you.