A wonderful thought! Let us know what you finally decide. Show us the finished product, please.

What a cute idea!!!!!!!! Here's what I came up with, Do the tights stop at the ankle (no feet)? To get more mileage out of them - and so that you can make a blanket for your DD too - trace them on a "shadow" fabric for the back. Draw feet or ballet shoes and place them in a ballet position. Position 1 on front and position 2 on back, for example.
Then you can make the same blanket for your DD out of the other side of the tights. Just a thought.......
Whatever you do, even if you just incorporate a square of that fabric in your quilt - it's a very, very special thing that you are doing.

I think it is a great idea. Play around with them to look at different angles, such as bent over or kicking up a leg. If you find a posture you like, you can clip and stitch them into that configuration before attaching as an applique. Please post a pic when you finish.

Better still, could you de-construct a pair of shoes that the girls each wore and include them?

I think the idea is cute, however I wonder if it would be just as cute (and easier to do) with a lavender fabric, cut to shape. She would recognize the "idea" and backstory of the pants and "get it." It sounds like the real pants have been worn and washed to within an inch of usefulness. Not sure if I would want to work with that, particularly as a beginner.

Regardless of what you decide, I think it is a sweet idea.

I don't have any suggestions for you but I wanted you to know that it is an awesome idea. There are lots of good ideas here from everyone to help you decide what you want to do. Please show us when you finish.

Thank you again everyone. Such great ideas!!!!! I knew this was the right place to come for input. I am finishing up my school year and getting ready for our recital (10 day event) and dance nationals, so I am going to think on all this for a little. But, I will come back and update everyone. Thank you again from the bottom of my heart!!!!

I would make some 9 or 12 inch blocks and appliqué different dance motifs with that fabric and maybe some pieces of other costumes

I love GingerK's idea of doing little versions of the pants appliquéd of many blocks.

Love everyone's ideas! Another one to add in is to use each leg to make a roll pillow. Two girls, two pillows! You could even use an embroidery machine to add in a dance quote, a name, dates, etc on the pillow. Good luck with whatever project you choose. Very sweet and thoughtful.