As hard as the truth is, it is ultimately kinder than to let your children hope that eventually his work will bring him home again.
My mother introduced us to a wonderful man when I was 7. My sisters and I adored him. He was our knight in shining armor, and life always sparkled when he was around. Then he disappeared without explanation. When we started asking "Where's Wally?" my mother made up a story about him being in a bad accident and having to be in the hospital for a long time. Meanwhile, she was pregnant with my youngest sister.
We waited. And hoped. Everytime we asked, my mom put us off with some detail about why we couldn't go visit him in the hospital. So we waited some more. Meanwhile, my baby sister arrived.
After a year or so of waiting and hoping, my mother finally told us the truth – Wally already had another family, and couldn't marry her. They broke up. My heart broke – after all that waiting and hoping, I found out my mother had been lying all along. I was so sick with grief I couldn't go to school for several days. I'm 63, and some small part of me still grieves fore the loss of my long-cherished dream, and I still dismay over my mother's dishonesty, although I have come to understand her motives.
Please don't take the cowardly way out. Tell your children the truth they deserve. Be sure to tell them your breakup is not their fault (even though, in a way, they play a part in it.) They will grieve, but the less hope they invest in a future that won't come, the less pain they will feel in the long run. A goodbye from your boyfriend would probably be good closure, though very hard for all.