My son had the same procedure done when he was about 2.5 years old. I wish we had done it when he was much younger - I regret that I listened to my pediatrician who said to "wait and see if he grows out of it" and then, when my son had a speech delay, the speech therapist undoubtedly recommended it. They couldn't say it was the cause of his delay, but it certainly wasn't helping.
We had it done at Children's Memorial downtown. It took 10 minutes and he was just fine the rest of the day and thereafter. The worst part - more so for me - was that they put him under general anesthesia due to his age - there was no way a 2.5 year old will sit still to have his tongue clipped! They are pros, however, and this is a VERY common procedure. It will bother you more than it bothers her and you are making a great decision to have it done when she is so young vs. waiting.
Today, my son doesn't even remember having it done and his speech - and his tongue - are great.
Don't worry - it's a very common procedure and you will likely be surprised at how little recovery and how little a deal it ends up being.
P.S. my son is a Halloween-er too (3 this yr), so Happy Birthday greetings! And my daughter is 6 mos on 11/3. Having, fun, aren't you?!