I would suspect that if you ask for a hair test, he will immediately shave his head. Many addicts do. However, it is hair from any area of the body including pubic hair.
Do you understand that a hair test, is just that, and depending on the length of the hair will check the whole hair. Since hair grows at an average of 3/4 to 1" a month, the drugs will be in the air from the root out. So unless he plans on going cold turkey for at least a month or longer, it will show his usage.
Ask your attorney what he feels he can present to the court to show his inability to have custody 1/2 of the time. You say these children are small, then how will he personally take care of them since he will be at work. Maybe your attorney can show that day care is not the same as actual caring of the children as you do.
I am just drawing a blank on anything else based on what you have said. Stress to your attorney how important it is that your children not spend time with a using addict, on how bad it is for their own mental health.
Love and hugs,