Blowing on a child after reading

As salaam u alaikum

On reading a surah, dua or even after reading namaz, one blows wind on a child. Please advise on the following
1. The significance on blowing on a child after reading

2. How tothe right method of blowing on a child as many people have advised different ways of blowing

3. What is the best dua surah zikr to read and blow on a child
.
Jazakallah