They're using low compression (lower speed, kid friendly) balls, often they're the same colour as your standard adult tennis balls just with a small alternative

They're using low compression (lower speed, kid friendly) balls, often they're the same colour as your standard adult tennis balls just with a small alternatively coloured circle to indicate. https://www.tennis.com.au/learn/rules-and-scoring/10-and-under-tennis-rules/low-compression-balls https://www.tennis.com.au/learn/rules-and-scoring/10-and-und... Look at the box of balls at 1:30.