Chaning Nanny's hours - how do I go about it?

[PacificDogwood]

Ok, so I might be overthinking this a bit, but bear with me.

We have just employed our first ever nanny and I am a bit green as far as my duties as an employer go (have had a CM until now).

Nanny will start middle of November (this start date suited her and us) for 35 hours/week.

Now, DS4 won't start school for another 4 years, but when he does there might be a case for wanting to reduce her hours (if she is still with us). How does that work? Presumably I'd have to give notice of the change? Should this possibility go into her contract (which has been drafted but not signed yet)? Anything else to consider?

She has been employed as a Nanny/Housekeeper so I am actually anticipating that as the DSs get older her role will simply change, but I am wondering...

Any advice greatly appreciated.

At the point of your DS going to school the job could dramatically change. So if changing the hours, then I would view that as being a case of the current job being redundant and a new job taking it's place.
You may well find that 35 hours a week could be continued whilst your DS is at school, it's hard to know.
As she is being employed as a nanny/housekeeper it could well simply remain the same when DS starts school, just that during term time the childcare duties get smaller and the housework type duties increase.

Yes, nick, that is my anticipation, that her role will simply change as our needs change.

But, if we wanted to make a change to hours some time in the future, would we have to make provisions for that possibility in the contract? It seems a bit daft to have to make her redundant, just to then re-hire her for changed hourse ?

She may not accept a change in hours. That is where I see there being a problem. You are offering the job based on them working a certain number of hours over a one week period with daily hours agreed between the two of you.
It may be that in future you could agree to a different number of hours per day but overall the weekly hours would be the same.
If the weekly hours were to change, such as say dropping from 35 to 15, then that I feel is a significant change to the job and is no longer the job they agreed to work. I presume that in dropping the hours you would also be dropping the pay.

See ACAS: Lay-offs and Short Time Working for more about reducing time worked by an employee.