RO water is efficient in removing chlorine, lead, nitrates, fluorides, mercury, iron, and harmful micro-organisms. This makes the water filtered from RO filter completely purified hence it is safe to drink.
However, because it is completely purified, the RO filter also removes essential nutrients from water. This is the reason why people are skeptical about using RO water. It is better to replenish the nutrients through a child's diet rather than exposing him/her to unhygienic water.
If you have doubts, you can first get the water analysis done by officials. They will suggest you about the kind of filtration system you need. The three most popular filtration systems available are - Carbon filter, RO filter, and Distillers. Get your water checked and accordingly select the most suitable one.