shorts; knee breeches
Refers to short pants that end around the knee; often used for children's shorts or old-fashioned knee-length breeches.
In summer, I put shorts on my child.
Elementary school kids in the old days often wore knee-length shorts.
半ズボン is the common everyday word for short trousers in general, whereas ニッカーズ names a specific knicker-style garment gathered below the knee.
Compound of 半 (half) and ズボン (trousers, from French 'jupon'), literally 'half trousers'.