I have two cousins, each with three kids, who have great bodies. The wife of one of my other cousins, who also has three children, is drop dead gorgeous, with an amazing figure.
Also, former FIRM instructor Susan Harris, who has four children, and who is now at least in her mid forties, looks pretty good here (some might find her outfit immodest):
Exercise video queen Cathe Friedrich, who only goes by her first name, also has a child. She looks pretty amazing. (Warning, some might find her attire immodest.)
Now if Jillian Michaels doesn't want biological children, that's her choice. I think in her case, she has some issues being that she was overweight when she was younger. It sounds to me like she's afraid of losing control over her body.