How can I help encourage my kids get more exercise?

Exercise yourself. A parent, hopefully, will provide unconditional love for their children as long as either is alive; they help guide, advise, teach, nurture, set limits on behavior and activity, don't spoil them or baby them (even when babies, think of them as little people). You are the model from which they will develop (either by imitating or doing the opposite). Exercise? Sport? Group their age & peer pressure.