It all depends on what form these interactions take. If it's simply harmless talking, then it's no more cheating than if you stopped to chat for a moment in the grocery store. That said, if it's sucking up excessive time that should be devoted to your current relationship, or of course if the exchanges involve something other than platonic chit-chat, then there may be an issue. My ex and I have a kid together, and so stay in contact. Thankfully, my husband is secure enough to know that any interactions between us are in the interest of doing what is best for our son -- and my husband even seems to enjoy chatting with him himself.