He's painted you into a corner by making you think you need evidence of this and proof of that. When you get talking to Womens Aid, they will set you free. Meanwhile, here's some reality:
Nobody "takes children away". Yours could and will be considered in danger IF YOU STAY WITH A PARTNER WHO ABUSES YOU. Your biggest risk of losing them is, in fact, by staying together.
He doesn't get to decide which DC go and stay. Since he's violently abusive, he would not get residency and will be lucky if he gets unsupervised visits.
Your photos and the emergency log will support that he hits you. This will be useful in future disputes over access, but has NO legal influence on any divorce or separation agreement. You do not need 'proof' of anything to leave him, and to take your DC with you.
But you should see your GP asap, in order to have more evidence of violence in case of residency & access disputes later on.
You can call the police if you are frightened of him, and they will remove him. You should call the non-emergency number and ask for the recent incident to be logged. Speak to their DV advisor; they'll ask the right questions.
You are in a stronger position than you think. But try not to piss him off too much while he senses you're planning to go. He might get nasty, keep your head down as needed for your safety.
Btw, it's very okay to write as much as you like about what happened here. I've got to go to sleep soon, but will check back tomorrow. x