It's more common in children, yes. I've had this since childhood. Try not to worry too much, it's not a dangerous one :)
I don't know the statistics but I've had arrythmias for as long as I can remember. My first memory of my SVT was in elementary school. I remember leaning against my little desk practicing writing my letters when my heart would take off. It caused my body to shake against the desk so I would lean back hoping no one noticed. It didn't hurt but it embarrassed me. Now I'm 55 yo and still doing fine with my dancing heart. Perhaps her cardio will be able to suggest something to free her up from this tachy. I know of another young girl (about 13) that had PSVT and had an ablation. Now she can live a tachy-free life, enjoy sports and never worry about a heart event. Good luck at the doctor's. Write down any questions you have so you won't forget when you get there.