No need to buy fabric, check your cupboards and make it out of an old shirt. Recycle / Reuse..... they don't take much fabric and this is so satisfying. For the drawstring try using an old shoelace or the cord out of an old sweater. Or you can turn an old piece of clothing into the string.
Click this link to download quick and easy instructions on how to make a drawstring bag.
Suggestions for drawstring bags include;
Sports Shoes
School Shoes
School Swimming Bags
Travel Laundry Bag (a couple of these are so useful)
Mesh Fabric and you have yourself some vegetable bags for grocery shopping
The uniqueness of these bags means your bag is always easily identifiable. Make it bright enough and with any luck your little people will spot it before they forget it.