Here are the words that best complete the paragraph:
Riding a bike is the best type of exercise for kids and is great fun. Moreover, it keeps kids physically active. Children should be getting at least one hour of exercise every day. Unfortunately, many children are not getting enough exercise. Riding a bike would help kids start exercising early in life and may help them continue the habit for life.
(Note: The selected words "Moreover" and "Unfortunately" provide a smooth transition and contrast in the context of the paragraph.)