Also try advertising anywhere local, such as a newsagents/village store which does postcard ads.
Consider what happens during school holidays - do you need care then, would it be more hours?
Think about what you are prepared to pay. The hours are not long so it may be hard to find someone it would suit.
How will they get between the schools - will you need someone who drives? Will they need to use their own car? Offer to pay mileage, 45p per mile (this is max amount before the mileage payment becomes taxable).
If the person only has this job and if you pay under £111 per week, then you may not need to register as an employer. Chances are though that will not be the case, so look into how payroll works. You can use a nanny payroll company (they typically charge around £150 a year) or you can do yourself via HMRC online.
Agree a gross salary. Someone doing 15 hours a week may well have another job, or get another job in future which may affect their taxcode.
Termtime employees are entitled to paid holiday, 12.07% of time worked.
If 15 hours a week, 38 weeks, then I make it 69 hours holiday which can be paid split over each term or an amount per half term.