OP, first of all may I say how sorry I am about your diagnosis. That alone must be devastating, let alone putting up with a selfish husband who clearly has no empathy whatsoever. You must feel like you've been hit by a steam roller?
Can I ask, do you genuinely feel that he will step up and treat the children well, when you no longer can? Or do you think he's likely to neglect them?
If you feel it will be the latter, then I think you should not only seek legal advice regarding your money, insurance, etc., but also a divorce, as there is no way that he should benefit from all of your hard work, having treated you like this.
From what you've told us, I can just imagine him shutting himself away in his shed, and forgetting all about the children. I'm sorry, perhaps I shouldn't say that, but it sounds like as well as being selfish, he is burying his head in the sand.
I would suggest that you contact MacMillan, and ask for help and advice with regard to your children. Tell them that you don't think your husband will cope once you are gone, and you are frightened that your children will be neglected. I'm sure they have come across scenarios like this on many occasions, and will be able to help and give you some peace of mind.
Do you have ANYONE else that you can turn to, any siblings, cousins, friends? If so, then please don't be too proud to ask for help and support.
I would also suggest that as soon as you possibly can, you move all of your money to a sole account, so that he only has access to what he actually earns.
Have you told him that he's going to have to work full time during the week, as with treatment etc. it's likely you'll have to give up work sooner rather than later, and it's no good him continuing to bury his head in the sand, he's a married man with children, and needs to step up and deal with his responsibilities. In your position I would have no problem whatsoever doing this, and if he argues, and says he'll deal with things when he has to, that gives you the ideal opportunity to tell him that that is NOT ACCEPTABLE, and if he's not prepared to step up and do his bit, then he can leave, as he's no use and no fucking ornament!
Finally, please don't give up treatment. I realise this is all very hard to deal with right now, but please be there for your children for as long as you possibly can.
I'm not very religious, but I'm praying that your husband sees the light, and decides to step up and look after you and your children, while there is still time.
Sending you a big hug to help you through this.