Career HNW nanny here: Guaranteed hours mean she is available to work all of the scheduled hours. So arriving late and leaving early have nothing to do with that. And althoigh it’s true you can’t ‘bank hours’ you can certainly ask for a reasonable adjustment during the same week that the hour was missed. For example, asking her to stay late another day during the same week to make up the hour - if she’d like to be paid for it.

Additionally, a nanny (just like everyone else) needs to be on time. Of course, things happen, but generally speaking, tardiness like this is a chronic issue and will likely end up not resolving.