The only way to teach this is by example. If you are a contented parent, live in the moment, don't dwell on the past, but look forward to what the future will bring, they will grow up knowing that this is how to achieve happiness and fulfillment.
It's not really enough to just be calm all the time. Calmness isn't contentedness. What I mean is that I'm quite an excitable sort of person, and express my joy and sense of humour very openly, and am very often not calm! But I am contented, with myself and my life, and my future too. It's about not being afraid, and it's also about talking as much as possible, so that no one has secret fears that they feel they cannot share.
Now that I think about it, contentment is a whole-of-life issue. If something in one's life is out of balance, it can affect whether or not that person is contented. So I would try to teach my children that contentment can come and go, but that it is important not to despair when it leaves them for a little while because it will return if they take steps to help themselves find it again. It's quite an abstract term really, 'contentment'. I've had it for a long time now, but it took me a very long time to find it.
Self-confidence is key to contentment, and parents can do an awful lot to help children find that through positive reinforcement. My eldest child has been bullied at school, on and off, for four years. But he has tremendous self-confidence, because I have always talked to him about how special he is, how talented, how kind, how worthwhile - and how brave and strong he is for dealing with the bullying so maturely.