Hi Daniela,
There is information available online about this. These links may help:
http://www.haringey.gov.uk/sites/haringeygovuk/files/how_to_self-administer_a_childcare_voucher_scheme..pdf
http://www.childcarevouchers.co.uk/parents/parents-how-it-works/parents-for-you/parents-salary-sacrifice/
Within Clear Books Payroll, you would put an employee on 'Salary sacrifice' (tick that box) and enter their 'Nominal annual salary' (the year's salary pre-tax) in the Pension details tab, then add the employee's salary minus the total amount of the childcare vouchers in the 'Gross annual pay' box.
Employees> View> [Employee name]> Salary details / Pension details
You should then create bills to the employee's childcare provider, reflecting the amount on the vouchers that you will give to the employee. You should pay the childcare provider directly.