If the nanny is being paid for that week because the MB gave her guaranteed hours then it is entirely appropriate for the MB to ask the nanny to come in if the plans change. The principal of guaranteed hours is that IF the nanny is available to work but the family takes extra time off the nanny will not be short pay. The nanny must be available to work for this arrangement to be fair. Guaranteed hours are not about maximizing extra paid time off for the nanny.
If the week off was scheduled as the employer's week of choice as part of the nanny's two week vacation and there was a notification time in the contract, then it wouldn't be appropriate. If there is notice period for the employer or nanny for their week's choice, then the MB does have a right to change the plans. The nanny needs to suck it up. I've heard of quite a few nannies trying to pull switching their vacation at the last minute hoping the family will not want to change their plans and she'll get an extra week off anyway. This is why a 2 week to 30 day notice period is good for both parties.
I don't think giving the nanny time off in January is an even swap. The holidays are now, and families are spending time together now. I would be upset that this change was occuring with less than a week's notice, and if the week off was part of my contracted vacation time at my employer's discretion I would be extremely upset at the lack of notice for changing what is just as much my vacation as it is theirs.