We had this a couple of years ago, my dad told us he'd been married before, but did have a child, we wasn't impressed (not the marriage bit, but the child bit). If you tell them, I wouldn't make a big deal of it, I imagine a lot of their friends at school may have step parents, so will probably not give it much thought.
These days information is easily available online, a quick search on Ancestry will show your marriage, if the kids do a school project on their family tree it may well throw up depending how common your name is.