I am currently taking Allegra 180mg once a day for my allergies. I've been taking it for almost a year and it works great for me.
The only side effect I notice is occasional headaches, but the benefit outweighs the side effects for me. I have to take an antihistamine daily because my allergies are year round and pretty bad.
I was previously on zyrtec and had to discontinue because it made me VERY depressed and tired. I couldn't stand how I felt on it(I don't know if you can equate this with behavioral problems in a child). I have none of this with Allegra. I am also on sudafed as needed and Rhinocort and doing allergy shots.
I am an adult so I don't know how it affects children, but for me it's great and I don't feel any different when I take this, like I did with the Zyrtec.
Just my 2 cents, hope this helps!