Term time vs holiday charges for holiday

Hi, I have a dilemma re charging for my mindee's holidays. I am new to after school pick ups and just approaching my first holidays.
My contract states that I do not charge for any holiday I take but that any holiday they take is charged at full price. I do afterschool pick ups and I also look after these children in the school holidays. My contract has term time times (e.g. 4 hours a day) and school holiday times (e.g. 10 hours a day). Obviously the parents want to take holidays in holiday time. I have told them these will be charged at the holiday rate e.g. 10 hours, rather than the rate they pay for term time. Is this too harsh? It is in the contract but they didn't seem to realise it and when I pointed it out for clarity, they were surprised and though not complaining they're obviously not happy. Do others charge term time rates if they take holidays in school holidays? My argument would be that the place could otherwise be given to someone else needing holiday cover but I would rather go with whats normal and fair. Advice much appreciated!

You say the parents have already signed the contract. When discussing the contract did they say they would need holiday cover? And I'm assuming that is when you said '10 hours'? Well that's what you charge. We all learn as we go along. Next time stress it more that this will happen.
I'm at a stage now that I don't want/have room for 'after school' children during the holidays. So everyone is on a term time only contract and don't pay anything over the holidays.
That is my choice and suits me. I budget accordingly. However, I have often helped a parent out and taken their child during the holidays, normally on a teacher training day that a parent forgot about, and charge them the hourly rate that my lo's pay. 'After school' rates are roughly an hour more because it's shorter childcare, school pickup etc.
It's your business, run it as you choose.

I'd also add I don't think I would get away with charging full in the school holidays, whether they attended or not, as there is alot of competition in my area with 'half term' holiday club/scheme thingys at a much cheaper rate. They are popping up everywhere and that is where my 'schoolies' go.

Thanks for your feedback MOH. Its tricky. I did reiteriate that they pay me when they take holiday, and they did state different hours in their contract for term time and holiday time, but I don't think I explicitly pointed out that holidays in holiday time would consequently mean paying me for the holiday hours. Tricky as I don't want to lose them or have them request a change to a term time contract and ad hoc days in the holidays as obviously that's what might come next if they're unhappy. One of my parents actually told me I should take holiday pay for when I go on holiday, and its only when I said I felt bad doing that as they end up paying me for these long hours when they go on holiday in holiday time that she realised this was the case. Holiday time is when I make my money really so reluctant to change things dramatically. Hmmm