I just had a look on Amazon and picked some that had good reviews. I can't remember now what they were, but there is also a good article rating a bunch if you google 'best headphones for kids' or something like that. It was recommended to me to get volume limiting ones, which we did. But if you plan to only really use them on the plane, I might not because my daughter really couldn't hear through them (nor could we) with all the background noise from the flight. We had to download an app that overrides the volume limiting feature just to use them! If you plan to use them more regularly at home and other times when not traveling, then I would opt for the volume limiting ones though to be on the safe side.
At that age, depending on where you're going, you may find lots of other ways to entertain her too. We first flew when our daughter was that age and short haul. Books, colouring, lots of snacks kept her entertained and she fell asleep too. More recently (at 4.5) we flew long haul (11 hours) and having a tablet with videos and games was a lifesaver sad she barely slept at all!