Babysitting fees - 14 year old niece

[Paws81]

Just after opinions really.
My niece will be looking after my two DS (10 and 5) next week for 3 days. 5 hours day one, 6 hours for day two and three.
my Dad will be with her for approx 3 hours all days.
We have said that we’ll pay her for her time. Would you give a flat fee to cover for all 3 days or an hourly rate?
I don’t want to underpay her, she’s giving up her time on her last week of holidays.
All 3 days I’ll be home no later than 1pm.

My dd also 14 has been babysitting our neighbours this summer.
It's her first time, so she sent out a message detailing her rates this summer would be £5 an hour, and will increase as she gets more experience.

Agree an hourly rate. Be prepared it'll always be that or higher in future!