9 year old who steals from cupboard

[Dexterrocks]

Our 9 year old dd continually steals "treats" from the kitchen cupboards in the morning before we get up.
Both my children are very early risers (5am is normal) and are therefore allowed to help themselves to cereal when they get up. However our dd helps herself to far more: sweets, biscuits, partially eaten bags of crips, glace cherries, syrup etc.
We have made it clear that we know what she has done and told her off for it but she continues to do it.
We have deprived her of treats when our ds is getting them because she has helped herself to them before breakfast.
She is well fed and, as I say is not expected to go hungry until we get up.
Any suggestions of ways to stop her doing this?
We have tried hiding all the treats in our bedroom and that is when she started helping herself to baking supplies.

[Readyisknitting]

I'm bumping this thread having discovered ds has stolen yet another chocolate bar (which I'd stupidly left by my bed having not fancied it). He'll always own up (having discovered the consequences for lying) but I cannot fathom this stealing. I ask him why and am met with a shrug. I used to do it, exactly as people above said, and ended up with usual teenage diet issues, plus really rotten teeth.

Ds is 7, and began to do this when we fled xh, and he is still doing it. He has a good diet put in front of him, but is so bloody picky, and I refuse to cook different food for him, so tonight was cottage pie and he didn't touch it. I do get that food is something he can control, esp given the emotional crap he still gets from xh, but he cannot hide behind that. I like the snack box idea, but that becomes a lot harder to do when you factor in dbs who is here half a week, and dd's 1&2. So ideas, please. I cannot lock everything- I've had packs of bay leaves disappear at his hands fgs.