Getting there is easy - Nippori Station is located on the JR Yamanote Line, which does a loop around many spots in Tokyo (so it's likely that you'll have a stop on the Yamanote Line not too far from where you're staying). Once you arrive at Nippori Station, it's only a couple of minutes walk from the station until you reach the main textiles street which stretches on for about a kilometer.
You'll know that you're in the right spot when you see the streets marked with these flags saying Fabric Street!
You can find all manner of fabrics here - from the traditional to the modern. Personally I love a lot of the fabrics that have Japanese inspired designs on them, like daruma dolls, kokeshi dolls, and cherry blossoms. They would make really cute clothes for little ones - I'm imagining little skirts with matching headbands using some of the fabrics in the picture below! There are options galore, with over 80 different retailers condensed into this one area - so you're bound to find a textile design (or multiple!) that you love.
I hope that if you're into crafts or DIY projects involving textiles that you'll have a chance to visit this area during your trip - there are lots of bargains and lovely fabrics to be found. Nippori is only a couple of stops away from Ueno Station, and there is a lot to do around the Ueno area (shopping, Ueno Park, multiple museums, and the Ueno Zoo for instance) so it could be a good area to plan a day around!