Well I don't have kids with my ex and neither does my fiance with his, thank god. However, my ex was very perturbed about me getting married again. He would make nasty little comments about having a "rich guy who could take care of me" and stuff. As if. He still calls me to try and get me to come visit my ex-parents-in-law, and tries to make me feel guilty by asking me why I never call them and telling me they miss me and junk. Argh.
The key is just ignoring the negative stuff. Exes are always jealous. They may not want the person back, but they always want to make sure that their former SO hasn't 'traded up'. It's a silly ego thing.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi