Nowadays, everyone speaks of yoga, which always was there since ages. I want to know that is it good for my 5 year old to do yoga. Is it of any good if my kid is doing enough physical activity required for her age?

It's always good for kids to be active -- keeps them happy and healthy. There are yoga lessons for kids as well as parent-children yoga classes where you can have lessons with your kids. It's pretty fun! Here's a video of such a class: https://youtu.be/-qBOdAde-mg