1. Unless you hold there is a prohibition of celebrating a birthday, why not celebrate her 12th. It’s a birthday party.
2. For boys, the ceremony involves doing things a boy at 13 can do which he couldn’t previously, but is limited to things that are not dangerous (having an aliyah and leading a minyan). Most other things one can do as a Bar Mitzvah (sign contracts, get married, be liable for criminal punishments, etc. – are not done for obivious reasons). For girls, the only big change is they can do all the “dangerous” things boys can, but which we don’t want them to. There really isn’t anything as 12 year old girl can do, that we want her to, that she couldn’t before.