請問 ... for child / for a child / for children ???

Hello, suggestions as below:

The one from you is wrong. Not complete sentence. --- "That book is not healthy reading for child / a child / children."

These are better:

1) That book is not a healthy reading source for a child.
2) That book is not under healthy reading guildline for children.
3) That content of book is not so healthy to children.

Thank you.

That book is not a healthy reading for children.
If that book is unhealthy, it is not suitable for all children to read, not only a child.