It is very important that your dogs get fresh air and sunlight every day.   This is were they get the vitamin D, which is important for proper bone growth and maintenance. 

From Wikipedia:

Vitamin D is a group of fat-soluble prohormones, the two major forms of which are vitamin D2 (or ergocalciferol) and vitamin D3 (or cholecalciferol).[1] The term vitamin D also refers to metabolites and other analogues of these substances. Vitamin D3 is produced in skin exposed to sunlight, specifically ultraviolet B radiation.

Vitamin D plays an important role in the maintenance of organ systems.[2]

Vitamin D regulates the calcium and phosphorus levels in the blood by promoting their absorption from food in the intestines, and by promoting re-absorption of calcium in the kidneys.

It promotes bone formation and mineralization and is essential in the development of an intact and strong skeleton. Although, at very high levels it will promote the resorption of bone.

It inhibits parathyroid hormone secretion from the parathyroid gland.

Vitamin D affects the immune system by promoting immunosuppression, phagocytosis, and anti-tumor activity.

Vitamin D deficiency can result from inadequate intake coupled with inadequate sunlight exposure, disorders that limit its absorption, conditions that impair conversion of vitamin D into active metabolites, such as liver or kidney disorders, or, rarely, by a number of hereditary disorders.[2] Deficiency results in impaired bone mineralization, and leads to bone softening diseases, rickets in children and osteomalacia in adults, and possibly contributes to osteoporosis. Research has indicated that vitamin D deficiency is linked to colon cancer; conflicting evidence links vitamin D deficiency to other forms of cancer.

Vitamin D₂ is derived from fungal and plant sources, and is not produced by the human body. Vitamin D₃ is derived from animal sources and is made in the skin when 7-dehydrocholesterol reacts with UVB ultraviolet light at wavelengths between 270–290 nm.^[4] These wavelengths are present in sunlight at sea level when the sun is more than 45° above the horizon, or when the UV index is greater than 3.^[5] At this solar elevation, which occurs daily within the tropics, daily during the spring and summer seasons in temperate regions, and almost never within the arctic circles, adequate amounts of vitamin D₃ can be made in the skin only after ten to fifteen minutes of sun exposure at least two times per week to the face, arms, hands, or back without sunscreen. With longer exposure to UVB rays, an equilibrium is achieved in the skin, and the vitamin simply degrades as fast as it is generated.^[1]

This is especially important for growing puppies.  I don’t supplement Vitamin D, I just make sure that everyone gets out in the sun when it is out.  Of course, this is no problem in the summer, but in the winter we have to take advantage of sunny days.

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