These foils for kids are made from size 2 or size 0 blades.  Size 2 blades are approx. 32 inches long and are good for children under the age of 10.  

Size 0 blades are the shortest (and lightest) and are 30 inches long.  Size 0 blades are also good for children under the age of 10, especially if you want a very light foil for them to use to gain more confidence in their ability to wield a fencing foil.