How would you react?

As a semi-religious person I admit that I would be disappointed. However I would realize that it isn't my decision and would try to be supportive.

it's a phase lol

Suppose there isnt really much you could do

Yeah, well, IMO it's better to support them.

I mean, It's their life, their decisions, I can't do anything about it.

As someone who has had religion forced down her throat every day, I can't see how attempting to force anyone (related or not) to conform to your belief system can end well. I can act the part, and I do every day, but someday, I will be free to be the real me, and it's likely that will be the last day I speak to my parents.

It seems like life in a family would be so much easier if everyone just acknowledged that religion is one of those individual things that everyone needs to decide for themselves.

I don't know how I would react.

I probably will ignore it. I am not religious, so I probably won't care.

If my child wanted to convert to Islam, I would sit down and explain how our ancestors fought and died to keep us free from forced Islamic conversion and to let us profess our own faith without and opression.

Honestly, whichever religion is okay, but if my child wanted to embrace Islam, I would be worried. I would still love my child, but highly dislike that decision.

As an atheist I wouldn't mind if child is religious or not. All I care about is giving him/her all possible knowledge and guidance to make an informed decision.