Definitely find yourself an experienced solicitor.
Keep a secret diary of any episodes of his anger/shouting/swearing/breaking things (preferably somewhere away from your home e.g your locker at work or a friends house etc), so that you can evidence his abusive behavior if necessary.
I think your H is assuming that he will 'demand' whatever he wants from you, and that you'll just 'give in', because that's what usually happens, and he's not going to be happy when that doesn't happen.
You need to be safe first and foremost. So quietly and secretly, gather up birth certificates/passports/ copies of financial stuff e.g bank statements/ investments/ pensions etc for both you and him (so he can't hide money or say he doesn't have a pension etc) and keep them safe somewhere away from home too.
Pack a 'go bag' with clothes/medication/phone charger etc for you and the kids and keep it hidden, maybe in your car boot or somewhere similar. You may need to leave in a hurry if he becomes angry at some point, and you wouldn't have time to grab what you'd need.
I'd also refuse to agree to anything he says. Just be non-committal e.g ''I see'', ''really'', ''I'll take that on board'' or ''I'll mention it to my solicitor'' etc because you don't want any 'escalation' of his abuse, which is what can happen whenever you actually try to separate. (Though you'd be amazed at how many of these abusive 'men' actually believe that your solicitor will see things his way 
)
Phone Womens Aid for more concrete advice. They are the experts in escaping from abusive relationships, so they'll be full of good advice.
I'd not worry at all about the threats of 50/50, because these 'men' don't want to do any parenting, never mind 50/50. He's just trying to reduce what money you'd get from him, but that's best left up to your solicitor so agree to nothing he 'suggests' or 'demands'.
You will also be entitled to benefits. Make sure you have the Child Benefit going into your bank account (open a new one if you don't have one he might be able to access by 'guessing' your PIN etc).
Apply for Child Tax Credit/ Universal Credit/ Housing Benefit etc as a single parent i.e separated, but still living in the same house.
Also do the Child Maintenance calculator to see what he'll likely have to pay you. He may well also have to pay you Spousal Maintenance as your ability to work is impacted because you have the kids.
Take things one step at a time (I'd start by setting up an appointment with a divorce solicitor and contacting Womens Aid , but be sure and do both when he's not able to hear you, so maybe when you're out of the house).
Good Luck 