I learned this from my Nana - Flannel fabric is the best for everything from dusting to cleaning bikes in the spring. Once a child has outgrown the PJ's or an adult has just worn through them, cut them up in various sizes. I cut the sleeves off and sew the ends closed to make for an easy dusting mitt.
I have the same idea but I use cheap queen size flannelette sheets from the charity shop snipped and ripped into squares in a variety of sizes for a variety of jobs. A $4 (Australian) sheet can supply me with 50 cleaning cloths. No need to hem, can be washed and reused or just thrown away after 'dirty' cleaning jobs.