Shoes
Indoor court shoes, not running shoes. The lateral cut and gum-rubber sole matter for the side-to-side movement.
Buy at the end of the day. Socks the kid will play in.
Used indoor shoes are not a great buy. The sole compresses with use.
Knee pads
Sized small/medium/large. Should sit just below the kneecap when bent.
Two pairs is standard. They take a beating.
Ball
Size 5 (official): ages 12+.
Lighter youth ball for ages 8-11.