My husband separated a few months ago. Originally he told me I would do the taxes and he only wanted 10%. That didn't happen I found today. He filed married but separate and claimed all of the kids. 2 of them are ours and the other 2 are mine from a previous marriage. Can he do this?

He already did it.

Is it legal? Depends on the level of support he provided. If he can show that he provided over half of the financial support for each child, then he has every right to claim them.

If he did not provide that financial support, then you need to challenge his claim with the IRS.