Link to the other message thread
As you are at the advertising stage, I feel you can advertise for exactly what you want. However, you may find that you don't get anyone applying.
It is a short period of time when they are not doing any physical work with your DD, so what would they do during that time... it may not be that practical for them to go home and come back again. It may suit someone but I would have thought that it will not appeal to many nannies, I know I wouldn't consider it.
There are some other things to consider which may or may not complicate things:
Holiday Entitlement - would need to be done in Hours, rather than Days, as the nanny is working a split day... so in theory could take morning or afternoon off, or both.
Travel to/from Nursery - I feel that the nannies travel time from Nursery back to your home, and from your home to nursery should be included in their working time, even though DD isn't present on those journeys.
During nursery holiday periods nanny would still be working the split hours, which could then affect possible outings for your DD. For example it may not be worth going to a museum as the amount of time there after travel may not be that much.
Keep in mind that nannies are not paid an hourly salary as such... like many other employees they are paid an annual salary which is split up into Monthly or Weekly payments and may be expressed as being a Per Hour rate.
Try to focus your mind on the annual cost, rather than hourly cost. That way you won't forget about things like holiday entitlement, how things like sick leave would affect things.
If you need it, this message thread includes details of many of the costs involved in having a nanny.