Part of entering a relationship with someone who has children, is accepting the fact that you will always come second to their children. If your partner has never given you any reason to believe he would stray, or any reason he would still harbor feelings for his ex, you should focus on that. It is a good thing for that child to see its parents get along, and show up for important events together, even if they aren't in a relationship. I think if you keep it in mind that they are going together to show their child they can be at the same event without an issue, you should be OK.
Realize that jealousy is normal and natural, and that everyone feels it. But also keep in mind that while feeling jealous is OK, the way we react and behave because of that feeling can be harmful and unhealthy. Keep your focus positive, and place your energy into the positive aspects of your relationship and your life. Be thankful for what you have, and if you can't move past these feelings of jealousy, try talking to your partner about it and see if he can reinforce those positive feelings you have about the relationship.
"Caring is not an advantage."