They are easy for the kids to put on themselves, with quality velcro straps and very strong PVC shell plastic to protect their little knees, hands, and elbows from getting scraped (or worse). In my son's medium set (ages 7-13 years), the knee pads were larger than the elbow pads, however in the small sets (ages 2-6 years) they were the same size so perhaps interchangeable? I couldn't figure it out.
We found the elbow and knee pads rather snug, even though our kids are on the younger end for the sizes they got, but better snug than too big! And they have nice breathable liners between their skin and the velcro straps. They also come in a great storage bag and come in 4 color options: black/grey, light blue/green, blue, and pink/purple.