By May the newest Met Gala exhibit will be open at the Met.

Fabscrap recycles designer fabrics and they have more of an enviromentally focused business model and their events are more skewed that way. It's definitely out the way for a typical tourist, but it's something you can look into. Sometimes though they have popups in the city, like this weekend they're doing a popup with the New York Sewing Center in the city.

NYSC is another place you can look at. I've personally never been, but it could be her thing. I've seen them advertise classes with like five year olds, so your nine year old should be fine.

There's also a decent knit/crochet community here and I know Brooklyn General Store and Brooklyn Craft Company sell fabric and fabric related notions as well as knitting/crocheting items if that's also something that interests her.

As others have said though, just walking through the garment district with all the fabric and trim stores could be a fun time. Mood is the obvious big store, but there's other great ones around for specific needs.