I consider 35-ish hours per week to be full-time and expect the commensurate benefits. This means that although I do have a second position for 1 day/week, I get my holidays, bonuses, sick leave, etc. from my FT family and they, in turn, get first dibs on schedule changes/my flexibility.
The problem with calling a 34hr/week job PT, particularly the schedule you're outlining, is that it doesn't give your nanny the opportunity to have another position that would offer benefits (or the opportunity to be enrolled in school, etc.) because it is so close to a FT position.
When you begin your search you are likely to find that the FT candidates are a significant step up from the PT candidates and will probably prefer to hire one of them. My advice would be to call this a FT position and give FT benefits for it to get the best candidate (and keep her committed to the position).