Is it illegal to deny medications for children that are clinically diagnosed with adhd?

No. According to amer acad peditrics practice guidelines for adhd, a doctor must recommend stimulant medication. The parents have ultimate say in what/how their child is treated. Having said that, untreated adhd is such a devastating, chronically pervasive condition that not treating it amounts to sentencing a child to an inferior life, increasing risk for drugs, motor accidents, financial problems.
Should refer. It is not illegal for a physician to decide they do not want to treat ADHD with medication, but they should refer you to a physician who can provide appropriate services. If you are asking about a parental decision, that is up to the parent.
No. It is more important to find the underlying cause of the adhd symptoms and treat them, making the symptoms resolve. If the cause of the symptoms are allergies then the diagnosis is allergies, not adhd.