Abusive men make you feel that it's impossible, they will find you, you'll be in danger, they will have people find you etc etc. But actually the help is there if you want to leave and want to make the best decision to protect your kids. Their life sounds awful - you said he's been horrible to you - the kids know this. Only you have the power to give your children happy memories. You need to seek help, be open, make a plan and leave safely. Even if you rang a women's charity and explained your concerns, the police can come and help you leave - he would be arrested if he tried to resist. Lots of things can be put in place to increase your safety. You might have to move to a completely different area and change your name - is that worth it for a new life for you and your kids? There is support for the children - counselling to help them through what they have heard/witnessed.
My relative felt exactly the same way, abusive husband she was terrified of. Subject to MARAC. Believed he would kill her kids if she left. She went to a (bloody lovely!) shelter for 6 months, had lots of emotional and practical support, divorced, changed her name, got a council house in a lovely area. Left with one bag and has built a life 10000000000 times better than she ever had before.
She resisted his attempts to get her back, nicely nice "I'll never do it again, I love you", "if you don't come back I'll kill myself", "if you don't come back I'll come and kill you - I know where you are" bullshit!! He ended up finding another partner and let her be. There is nothing that the women's charities have not seen before. They have helped travellers, people from Asian community where there are links all over the country and they have the ability to help them. Once you leave do not go back!
Nothing is insurmountable!! You do have the power and the strength to make change - he is not as powerful as you think - or else he wouldn't be abusing a woman!!