Keeping your children active now will benefit their bones for the future.

Weight-bearing exercise works to build stronger bones by stimulating cells responsible for the synthesis and mineralization of bone (osteoblasts).  As you put more tension on your muscles it puts more pressure on your bones, which then respond by continuously creating fresh, new bone. 
This is important at all ages, although there are certain periods in your life when exercising for bone strength becomes particularly important and effective.  Peak bone mass during childhood and adolescent years is one of the major factors that can either contribute to, or help prevent, osteoporosis down the road.  Researchers found that exercise when you're young leads to stronger bones, with the benefits persisting during aging, even into your 90s (1).  Boys and girls who were the most active throughout their lives (starting at age 5) had denser bones and better bone shape than less active participants at the age of 17 (2).  Specifically, half of the bone size and one-third of the bone strength built up by early-life physical activity was retained throughout life (3). In other words, physical activity during youth provides lifelong benefits to your bone size and strength.

Unfortunately, the study (1) found that most children and teens are not active enough to take full advantage of these formative bone-building years, especially as they got older.On average, girls spent just 47 minutes being active each day during childhood, and this decreased to 24 minutes a day at age 17. Boys' activity levels fell from 60-65 minutes a day during childhood to 36 minutes a day as teens. 
The key is to get your child interested in physical activity from a young age. Even when activity decreased during the teenage years, children who had been more active in childhood still had better bones.  Encourage your children to engage in activities that are naturally interesting to them, such as playing on the monkey bars, rollerblading, skateboarding, dancing, playing basketball with friends, hopscotch, jumping rope, or even helping you in the garden.

Karolyn Boyd

Nutritionist

Ottawa Holistic Wellness

References:

1. Warden, S.J., Mantila Roosa, S.M.  Physical activity completed when young has residual bone benefits at 94 years of age: a within-subject controlled case study.  J Musculoskelet Neuronal Interact. 2014 Jun;14(2):239-43.
2. Janz, K.F. et al.  Objectively measured physical activity trajectories predict adolescent bone strength: Iowa Bone Development Study.  Br J Sports Med. 2014 Jul;48(13):1032-6. doi: 10.1136/bjsports-2014-093574. Epub 2014 May 16.
3. Warden, S.J. et al.  Physical activity when young provides lifelong benefits to cortical bone size and strength in men.  Proc Natl Acad Sci U S A. 2014 Apr 8;111(14):5337-42. doi: 10.1073/pnas.1321605111. Epub 2014 Mar 24.