Children eat so much sugar these days and I honestly believe it's massively problematic on so many levels. I have friends who have cut the amount of sugar in their childrens' diets and seen massive positive behavioural changes and as a result have happier calmer children.
For once... I don't think the OP is being unreasonable. It's the fact her daughter took them without asking. Surely at 11 you ask? The issue is she specifically knew she wasn't meant to have them - she hid them, etc.
Now, I don't think it's healthy to have blanket rules like all snacks must be 'healthy' such as a fruit or yoghurt, and I do believe a sleepover should be an exception - BUT I don't agree with kids having sweets every day, biscuits every day. It's setting up horrible patterns and quite honestly it's setting up health problems and people sometimes think it's cool to be blind to that.
I think it's fine for them to have biscuits on special occasions but that's what it is - on occasion not every day...
OP I'd speak to her openly about it and ask her why she took them and hid them... maybe she is hungry. 11 year olds need a tonne of food.
When I was growing up I was very underweight and hungry a lot of the time. There was no malice or intent and my mum would be horrified if she knew that now so I would never tell her, but she simply didn't realise the amount of food someone growing or going through puberty needed, especially if they're really active. You know your own child - is she healthy and well?
You say if she'd asked you would've said yes, and there also was an issue with the DVDs, so it does seem to be more of an issue with sneaking things than to do with food... so I'd try to get to the root of why she is taking things. Does she have much privacy at home - able to lock her bedroom door, privacy in the bathroom, able to have a diary that isn't read? I didn't have much privacy and I actually think that's where some of my problems may have began, I don't know.
Good luck with sorting it out.