Someone correct me if my maths is wrong but I think
He - £1500 pcm pays rent at £900 leaving him with £600 purely disposable income
Op - has £100 left over after all bills and household & children's costs paid
There's a "joint" savings account that he never or very rarely pays into but withdraws from
Does what he pays in rent equate to the same amount as you're paying out in bills etc op (Excluding your personal non-necessity expenditure) INCLUDING the kids expenses (he is equally responsible for them)?
I suspect there's a huge discrepancy there, frankly this all sounds like financial abuse.
You say you couldn't afford to live without him but have you actually checked that?
Check what expenditure you WOULDN'T have without him (reduced grocery bill, reduced utility usage & similar and anything you cover that only he uses eg sports tv package type thing, his phone?)
Check what benefits you'd be eligible for and and things like council tax discount.
You might find, certainly if as hoped you get more hours in your job, that you could manage financially without him. Could prove enlightening and make you less willing to put up with his selfish nonsense!
You're doing amazingly well with all you manage. Nothing wrong with having kids young there's pros and cons to being a parent at all ages.