Oy! Boy oh boy!
If you were just dating casually and no one was coming around the kids, then he has no business in your romantic life. However, getting into a serious relationship and getting married? He needs to know and has a right to know who his kids are around. Even though your husband may be just dandy, the father of your kids needs to know these things. How is it that he doesn't? I would think your kids would have brought it up at some point.
But even though this is a long distance co-parenting situation you still need to communicate. Communication is the glue to any relationship and any parenting communication.
The only thing you can do is fess up to your ex and face the music. I am more curious as to how much of a relationship your kids have with their dad if he was kept in the dark for that long.
If there is no dad-kids relationship and he's completely MIA, then you don't need to say anything nor does he have to know about who you marry.