Hi! It's a tricky one and sorry I haven't read the full thread.
You sound a bit like my mum, in that her partner (let's call him bib; they never married, but he lived with us from when I was 10, and they were still together when I moved to uni age 18, though split perhaps 10 yrs after that) was very much 'her partner'. She tried to keep her relationship with him separate from our (mine and my siblings) relationship with her and discouraged any aspect of him acting as a dad.
None of us saw this as problem at the time, but looking back I think it's sad.
What would have been the harm on him coming to parents evening, showing an interest and supporting the family finances, including supporting me a bit financially?
I think my mum was worried it would be a burden on him to 'take us all on' and difficult for us to adjust to a new dad figure.
But the reality is it's uncomfortable having a separate person living in the family. It's a weird dynamic and uncomfortable for all. I expect with hindsight bib would have loved to have been invited to get more involved, to have had some financial responsibilities for us, to feel valued and included.
Your DH hasn't just married you, he has married a package which includes your kids. He should be more part of the family, and yes, that does include financially contributing to the family. Don't be scared of it. He won't run. Of you think he's the kind of man that would leave a marriage over this, he's not the right man anyway.
I think it could bring your DC and him together a bit, and perhaps be a symbol of how he could be more part of the family in other ways. He isn't separate now, he's their stepdad