I suppose the question is if you have personal bias against heavy metal or is it actually evil? I still believe that not all heavy metal is bad. There is some out there that is so awful. It is the lyrics and intention behind the song that makes the difference. I understand each person has their own likes and dislikes regarding music and that is okay. To some, country is displeasing and to others it's rock n roll or something else.
I find it interesting you mention kids here. Why only kids and not adults, I wonder?
Either way, we live in this world. We are called not to be of the world and not to follow the desires of the flesh, but to pursue the fruits of the Spirit and to please God. However, this does not mean we hide from the world or not be able to enjoy some things of this world.
You ask if there should be differences between what kids do in and out of church, but why? Should we hide the music we like outside of church? Or should it always be God honoring inside the church and out?
I will let you in on a little story. When I was younger, I had my own pop songs I liked. I had a tape of said music inside the car and would listen to it Monday-Saturday whenever we were in the car. Notice a day missing? Oh, Sunday mornings. On Sunday my music was shameful. My mother would say, "None of that music before church." And, "Don't talk about your favorite bands at church." I would lie when people asked me my favorite bands/songs. I was so confused how my parents let me listen to my music, even jokingly singing along and then the day church falls on it becomes shame. This is confusing to a child. So, where is the right in this scenario at all? Would it have been to disobey my parents and tell the truth? Would it be for my parents not to let me listen to music at all?
If we are doing something that we are hiding from our brothers and sisters in Christ, we need to evaluate ourselves carefully.