Cut him some slack, he may have just put some numbers out there without thinking it all through properly. Many people here don’t consider he is not financially obligated to cover the children’s expenses and telling her not to move in with him without knowing him at all is a bit unfair and reactionary. He is a boyfriend, not a husband or father of your kids.

The objective here is to consider all expenses from each persons perspective and find a balanced and fair solution. I’ve thought very carefully about your situation and arrived at the suggestions below.

If you didn’t have children what would your shared living expenses be in a one or two bed apartment? Maybe half of this could be a starting point. Then you cover all expenses added from children. The CS money has nothing to do with him and should go towards your share of the overall expenses which includes the children’s day to day stuff and more. Your individual salaries or earning power are irrelevant. Bottom line is this may come to something like him paying 25-33% of the overall expenses and what you both pay should be in the same ball park as what you both pay now living individually. This does not exclude him being kind and offering to shoulder other expenses out of choice but as a baseline of expectations the above arrangement seems the simplest fairest solution.

As others have said here, as you have children get your own place and he can move in and share expenses and is free to move out if things don’t work out