Regular Peltor earmuffs are suitable for most children age 5 and up. By adjusting the cups as high up both sides of the headbands as possible, the earmuffs should fits snuggly over a child's head. For your reference, the size of the ear-cushions is as follows:
Inside measurement: 2-4/10 in. by 1-4/10 in., oval shape
Outside measurement: 4-2/10 in. by 3-2/10 in., oval shape
However, if you need earmuffs for even smaller children or even babies, the you should use the Peltor "Junior" headset. These have even shorter headbands to fit over the smaller heads of very young children or babies.