I have to say that a household and environment that fosters a love of music will have a large impact on your daughters interest. Likely only piano lessons with an instructor will assess her interest in that particular instrument, but that's really just the tip of the iceberg. There are lots of other instruments, most just as fulfilling, as well as singing (oft overlooked) and guitar (both classical and popular/folk.) In addition, interest in music in general is (in my opinion) stronger and longer lasting than interest in any specific instrument. In that vein, here're my recommendations:
Definitely try to work some rust off, and get back to playing what you can, but also think about a few other ideas:
- Have a household listening night just to listen to music, of all genres! It can be while playing a board game, even, but perhaps periodically stop to point out certain aspects of the music. Try to include lots of different types; popular and rock to classical and musical theatre if you can. Try to include everyone!
- If you become proficient enough at piano or guitar, or your S.O. or other housemate is, have a singalong. Folk tunes and traditionals are fabulous for this purpose, and it brings the family closer together in a big way. Once someone is proficient enough for accompaniment (any chordal instrument), check out the book "Rise Up Singing." Lots of material to choose from.
- If you can, expose your child to other aspects of culture, including live performances. Even high school and college bands will be great for older kids (say, middle school), but younger kids might need something with a little more "pep." It all depends. I live in NH, and there are quite a lot of summer theatre companies. They put on fantastic shows using professional and college actors, and the classic musicals are a heavy part of their repertoire.
I guess my main point is: be creative when trying to foster her interest in music. Don't go to the easy way out, which is just enroll her in piano lessons with a teacher, and hope it catches on. Too many kids stop once their parents allow them to make the choice, and then regret it ten years down the road. Ideally, these tips, and many others will make her love of music and culture so great she can't resist playing an instrument. Piano is the classic choice, but violin is a great starter instrument as well, as it comes in tiny sizes! Guitar too! Once she gets to the traditional band age, usually 4th or 5th grade, see if she's interested in that. And if the finances allow it, try not to limit her to the first instrument she chooses; it's not necessarily the best fit! (Rent first, buy later).
(I realize after writing this that I sort of drifted from your original question, which was whether to try and gauge her interest on piano yourself, or enroll her directly in lessons. In my opinion, gauge her interest yourself. But try to incorporate the bullet points above at least a little.)