Want to add a 2nd line to my auto prepaid acct but need an upgraded phone

I currently have a JPrime or something which is fine for me. My 12 yr old for her birthday wants something similar to a Galaxy S6, how do I buy an upgraded phone to add  as a 2nd line to my existing prepaid plan.The selection for prepaid phones @ at&t online are limited. Im not tech savvy so confused about options like unlocked, do I need  a$10 prepaid kit if I just buy a phone online, not thru at&t? Then Im scared the phone want be compatible. Does it have to be a "prepaid phone"? Ive read so much conflicting info Im even more confused..I dont mind spending a couple hundred bucks on a phone as her b-day present..As long as I only have to pay 30-40 bucks a month for her service..Thanks in advance for your help!