Having done term time only myself in the past, I have to say I don't like it as it complicates things. You may find that some nannies won't do a term time only job, others might but may want a higher salary to compensate for not being paid all year.
Issues are often around:
when holiday can be taken
how pay is split - working time or over 12 months
how much paid holiday they get
how much unpaid holiday they are expected to take
I would suggest going on the generous side... so giving at least 6 weeks paid holiday. So if they are doing a 38 week year... then you pay for a 44 week year. What you then do for the other weeks is up to you to agree.
I think the current way of working out minimum holiday is 12.07% of hours worked per year. Wrote this in 2011, think it is still current. Not sure Governement ever rewrote guidance on term time working.
As an example, a nanny working 30 hours per week would be doing 1560 (30x52) hours per year. A nanny working 30 hours per week term time only would be doing 1140 (30x38) hours. Term time weeks may not be 38, it can vary between educational establishments, plus teaching staff may work more weeks than pupils attend.
If a nanny was doing 1560 hours and had 6 weeks paid holiday, then holiday would be 180 hours. If gross hourly pay was £10, then annual salary £15600.
If a nanny was doing 1140 hours and had 6 weeks paid holiday, working hours would need to be increased by 180 hours (6x30) giving 1320 hours. If paid at £10 gross, then £13200.
As you can see, there is a difference in the annual pay... thus why some nannies won't do term time only, at least not at the same hourly salary.
Things can always come up during term time. So even where a nanny agrees to take their holiday outside of term time, there can be occasions where time off is needed. For example a relatives funeral. So you need to come up with a solution for that - which may be that you take a sick day from work, or whatever sort of late notice leave your employer may allow.