You have a right to the full story, and to be part of the "game plan". This is your home, and your family, too. I would approach the subject with him like a mutual planning and strategy session. Ask him to explain the whole story to you, and his feelings on what would be best for the kids, if he thinks they will spend time with their mom, or if it's out of the question.
You should absolutely prioritize taking care of yourself, too. You did marry a guy with two kids, which always entails the risk that you are going to end up parenting them full time. So I'm afraid you do have to understand. But needing a break isn't evil, and needing some time with your SO isn't something to feel guilty about. There's nothing wrong with making the effort to grab a week or so together while the older kids stay with their grandma. The craziness of blended families is REAL, so take care of yourself and your relationships!