Hello Sarah, welcome to the forum
I am so sorry to hear that two people loving each other couldn't find an acceptable way to work things out over an ex-partner and kids.
I cannot really answer your question though. You should look into your heart and see if on this cruise you two could end up staying together and work things out, or.. not.
If not, I would cut my losses and back off. The cruise will only hurt you (and him) knowing that things could be great but it just won't happen.