you could allways tell your child, NO, tv or computer games until he or she has done the excersise you choose for them. Even if its a ten minute bike ride or ronning up and down the stairs a couplr of times.
It probably all comes down to confidence for the child. Make is something 'cool' like karate and something that you progress quickly at, not too competative. Hopefully the child will feel motivated by a sense of achievement.
Hi do something with them, go swimming, ice skating, rollerblading they will have fun and so will you. I do all these things and more with my son, he gets to laugh at his mum and we are both exercising at the same time.