OP how much do you think he earns? How much assets do you think he has? Your children might be able to claim more than you think if he's richer than he's been telling you (which frankly is probably in this case, as why would he be so worried about you taking his business if it's only worth a couple of grand?)
If he earns around £150k gross, you will be able to use Schedule 1 of the children's act to claim child support, top up child support, carer's allowance, a housing fund (I.e. he will have to buy a house for you and your children until uni), private school fees and cars allowance.
There was a case where the dad (it's always the dad) had to sell his own house in order to buy a house for his ex and his children.
He probably won't voluntarily tell you the truth of what he earns, but can you do some digging and spying? Use company's house directors roll if he owns a business. Try to figure out what credit and debit cards he has, fuck it, have a look at his bank statements if you can!
Particularly to this sort of case, you CAN claim legal expenses from the ex too.
If he earns 150k, and if he can afford any/all the above, you will do better than the CMA minimum.
And ditto to other posters as to the custody. You need to mentally prepare for 50/50, but you are very unlikely to get there, and probably more likely EOW.
See a solicitor asap - don't tell him, and do some research on this law. You and your child/ren might be able to get a lot more than you think. Ideally do this now, so you can consider your pregnancy in light of what you learn.
Get mentally prepared for a long battle though, get tough. Get skilled up and get your own career.
You sound like a nice person, you will find it in you to get through this.