Please suggest me tetra pak milk is safe for a baby of 20 months old

While some web sites say it is OK, many say it should not be used for children, and never for infants. You did not say where you are from, if India, I could not find any Indian site that said to use it for small children. Here in China it is not used for children.

We have it in Canada. We have had premixed formula come in tetra paks and used it for our new born children. Look at the package, it says that it is pasteurized then you're fine. You're 1.5yo child will be fine if it is pasteurized.