I agree with previous posters who suggest you need to think a bit more about your relationship with this man. If you are so negative about a key aspect of his behaviour (parenting) within a short paragraph, I wonder if you are positive enough to be marrying him or forming a long term commitment. Think carefully about this and about how you need to feel about a man to marry him - it's not that you have to think he is perfect, but you must have respect for him and any issues to be pretty minor. Your issues are not minor.
Secondly, if you are marrying him, you are also taking on the children too. You won't be their mother, but they are a key part of his life and need to be a key part of yours too - and that will mean some paying for things for them. A determination to split everything 50/50 also suggests to me that someone isn't ready to fully commit to a relationship but wants their own autonomy still. I'm not saying you must have shared finances, but being open about finances is a key part of a good relationship and some give and take about either of you sometimes paying more and sometimes less is important. It is impossible to act as if the children don't exist in financial or other ways. Their existence will impact your relationship and you personally on all kinds of levels....and you have to be prepared for that if joining your lives together. If you're not up for that, a man with children isn't for you.
It sounds like you feel he is after you paying for his kids.....if you do feel like this, again it rings alarm bells about the nature of your relationship. If he is doing this, it isn't great.....and if he isn't, but you perceive it as such, it's a problem too.
More communication is clearly needed.....about the children, his parenting, your relationship and money too are needed. Have these conversations before moving the relationship forward. It will be better for you, and there are kids involved, so moving forward in relationships which don't have the basic building blocks for success is clearly not great for the kids either.
Good luck OP in thinking about all the aspects of your relationship.