The main cause of rickets is lack of vitamin D or calcium in a child's diet.
The lack of Vitamin D and Calcium causes bones to lose strength and then can cause the bones to become brittle and unhealthy and can lead to the rickets which can also cause bow legs and slow growth.
Rickets are not as common today as they used to be years ago but one main reason that kids could develop rickets is due to lack of sunlight which is a main provider of Vitamin D.
Milk provides your body with Calcium as well so make sure the kids and yourself get some milk if you can drink milk and get plenty of sunlight to prevent rickets.
Not eating a healthy diet or going without food and becoming too skinny and underweight can also lead to rickets as well but the main cause like I said above is the lack of Vitamin D and Calcium.