Ok miss, I'm going to tell you to BE VERY CAREFUL from now.
By saying you are leaving him, you leave yourself and him to a LOT of raw emotions, and things are going to develop very quickly, and not necessarily in your favour.
If you are absolutely sure you want to leave him, you need to find a solicitor NOW, and pay them to get full financial disclosure started TODAY. Any extra time he has will be time for him to hide his assets (if he hasn't done so already) or pass them to his parents.
Do you know how much the company has been paying him per year?
If you claim on schedule 1 - they DO take into account dividends and non-earned income, so if he is receiving that you should be fine.
However, I suspect all the dividends etc are going to his parents, and his parents give him money as 'gifts', for tax reasons, and going by the ownership of the company. which unfortunately means you and your kid/s might be royally screwed. But you won't know any of this unless he tells you, so how you manage to get this (lawyers, pretending to play nice again whilst biding your time, getting your hands on some paperwork...) is up to you - you know better than us what might work.
If it turns out that he 'doesn't' earn or own very much, whether he is hiding or not, then you need to convince him that he needs to do his best to support his children, and their carer, and give more than the minimum child support. Again how you do that you know better than us - whether you can get him to see sense.
He will have been hurt by your refusal to fall in line, and i suspect he will want to punish you the only way he knows how - withdrawing financial support.
I don't know what will happen, and I really hope it all works out for the best, but be careful here.
The only thing you can do is prepare yourself - get a career, understand your legal position and his. You can't control him, but you can control yourself and your actions. You will do better if you are calm, cool heading and forward thinking.
Wish you all the best.