I usually have repeated contact with the parents before I meet the kid. I introduce myself for the first time as Dr. Firstname Lastname, and give the parents my card with my credentials on it. In subsequent phone messages (the only time I continue to refer to myself), I say Firstname Lastname a couple times, then just start using my first name. Many families continue calling me Dr. Firstname or Dr. Lastname after I'm using my first name with them, and some just use my first name; both are fine with me. When I meet the kid for the first time I say "My name is Dr. Firstname Lastname, and you can call me Firstname."
For what it's worth, I do the same thing with adult clients- full title the first time, then gradually walk back to using just first name, and let them call me whatever they want.