Hi - wanted to comment from 'the other side of things'. Am going to qualify this a bit - this is a subject in which I'm massively emotionally involved, so the following might sound a bit impassioned/dramatic...Apologies if it's a bit over the top but feel v strongly.
I'm now 30 - my mum has had bipolar and anxiety disorders since before I was born. She took the decision not to tell me about this during my childhood and adolescence (I eventually had to find out by writing down the names of her medications and looking them up in a BNF in the local library). I think she was probably trying to protect me, but it had totally the opposite effect. I spent much of my childhood anxious, nervous and consumed with worry about my mum - I would lie in bed at night wondering if my parents were divorcing, if she was dying of cancer, and things came to a head when I came across a half-written suicide note left in her bedroom. In the way that children do, I also struggled to explain mum's behaviour to my friends, worried that I would get teased if she spent the whole day in bed crying whilst they were round, and generally felt totally deskilled in dealing with what was going on at home. I ended up self-harming just to relieve the stress, and spent 6 weeks wondering how to explain to my friends she was an inpatient in a psych unit, when I didn't know why myself. The lack of knowledge coloured my whole childhood - I am a natural worrier/neurotic anyway (always have been!)- and I feel I was a bright child who could have coped much better with the truth than with being 'protected'.
There's probably an element of personality in this, and of course you know your dd best, and how she is likely to respond. But I think your dd will probably already know some of what's going on, and I strongly believe in being honest with her. Let's face it - if you'd broken your leg or had arthritis you wouldn't hide it from your dd, and mh issues are health problems like any other.
Hiding this from your daughter also suggests that mh probs are something to be ashamed of/taboo, and its a shame to give your dd that message, even if it's not deliberate.
On the subject of seeking support from your dd though, I think this can be dangerous ground. Weirdly, altho' my mum wouldn't tell me what was going on, she did seek a lot of support from me in ways I now feel was too much for someone of my ag, eg. 'you can't go out tonight because I can't cope'. I was sorta torn between being a child and a carer and ultimately felt v responsible for her. I think it's fine to ask for your daughter's understanding and support to an extent, but that you should ensure you have other avenues for this too...eg counsellor, partner etc.
Hope this helps a bit. Sorry to ramble and don't mean this as any kind of attack on you. Just a subject close to my heart...
It will be interesting to see how I fare. I am still in treatment for pnd, and have had 2 x episodes of clinical depression prior to that. My dd is 18m, so too young to ask questions or need to know at present. But think I will be confronted with the same dilemma v soon...
All the best