How can we possibly know whether it could work out or not with your nanny? We have no idea what her home set up is like or anything. If she has a partner and would like to work part time this could be brilliant for her, if she's on her own with a mortgage then it's highly unlikely she can go down to part time hours.
But honestly, you really don't even need to think about this for another year. It's a LONG time away and anything could happen between now and then.
Why can't you afford her full time when your kids go to school? Will you be earning any less? Do you mean you don't want to pay her a full time wage when the kids are at school? Which although in someways is understandable, it's not the same thing.
I don't know how many kids you have - but wrap around childcare & holiday care for more than 2 kids is probably not going to be that much different.
Do you want to split all your holiday into small bits to cover holidays, sick days, inset days, snow days?? Or do you want to keep paying what you are already paying and take proper holidays with the kids?
Why not talk to her, see what she thinks, if she could consider part time once your youngest is settled into school (still think you'd be mad to cut her hours before your youngest has been at school at least a term - if at all).