Don't force them to do something they don't want to. If they don't like bikes they don't like bikes and no matter what you will do they will avoid riding. Try to find out what they like so it will be enjoyable.

I also understand the whole dilemma of you can't workout because your fat and your fat because you can't workout. If you are overweight you may not mind going to a gym to lose weight but then there is always the embarassment and looks.

Current age, weight, height, fitness level and interests will help find a suitable activity for weightloss.

Also don't forget diet is a LARGE part of this (bad pun I know). I can ride my bike 100 miles and lose no weight if I eat poorly and I can lose 10lbs a month and never ride if I am on a very strict diet. It is all about compromises but still essential to get a good diet now.