I would ask first. Different priests have different feelings about this.
A few years ago, I was asked to take some pictures during Mass by my kids’ school. I asked the priest and he had no problem with it. However, I did not want to be a distraction, so I turned off the flash and simply took pictures from my pew. There were a few pictures that weren’t too bad, but without a flash…
Our current pastor does not allow picture taking during First Communion and the like. There is a professional photographer hired, and he is the only one taking pictures.
Definitely, though, try to get a few pictures to mark the occassion - maybe before Mass when she is dressed in whatever altar servers dress in at your parish. Or after Mass as she puts out the candles (or whatever). A picture with her and the priest before Mass.