This situation is a disaster-in-waiting for you, for all the reasons listed by PP.
Step 1: Go fulltime at work. You need to do that immediately. You and DP will split the cost of childcare by % of income. So, if he earns more, he will pay a higher amount.
Step 2: Regardless of pay, your job is as important as his. Indeed, moreso, as you have lost out on promotions, training, wage increases, pension contributions. So you two make a plan for when dc are ill or childcare falls through. It will be evenly split between you.
Step 3: You working f/t has an obvious impact on housework, shopping, etc. Work out a plan to split this evenly between you.
Step 4: Work out the financial reality of leaving him. What can you afford in rent, or can you afford to buy? If so, where?
You don't have to leave him if you do not want to. (I mean, he's lied to you about the foundation of your relationship for years: I'd leave him.) But you must start living as though you are single. Because you are, financially.
I know that you wanted the life of a SAHM, at least for a while, which is a perfectly valid life to want. But he doesn't want to make the legal commitment that kind of life requires. I'm really sorry it didn't work out.
But you are young and you have an easy pathway to full employment. It might not be what you wanted, but you can make it wonderful.