I have bi-polar as well as Crohn's disease so my kids have seen me really ill (2 boys - 11 & 14 now).
I explained my bi-polar to them along the lines of being like they feel with their hormones/puberty.
Along the lines of:
"Sometimes I'm really, really sad and crying for no reason at all - it's nothing anyones done, and nothing can change it - it just 'is'.
Sometimes I get angry quickly, but it's not anyone or anything that's made it happen and I can't control it but I can try to 'manage' it, just like they have to."
They're ever so good about it & I think that because they 'know' it makes it easier for them to process and understand - sometimes when I start ramping up into hypo manic they notice before I do and they'll mention it to me.
Wrt your husband, it will get easier, the hardest part is making the initial break and starting things on your terms.
I've been separated a year now & I'm fine, I was so scared that I wouldn't be and my mood has rapid cycled a lot, but I just take a day at a time when it gets really hard.
I've been quite open & honest with my boys about me and their dad - I've made it clear and reinforced that my relationship with him is completely separate to their relationship with him - although I may dislike him at times it doesn't mean he's a bad person etc.
It sounds so glib just typing it like that, but it really has been ok - you know your own children & you'll know best what level of detail or information they're able to deal with.
Be kind to yourself though, and don't put too much pressure on yourself to somehow magic everything better because you'll just be setting yourself up to fail in the long run.
Lots of love and hugs - it's hard, but it's possible - small, manageable steps and you'll get through it all. 
