Red Balls are at 25% compression of the standard Yellow Ball and are designed for players aged 5-7 yrs old.

These balls are designed to land within the hitting zone (roughly waist height) for players in this age bracket

Typically these balls are larger than a standard ball and thus easier for players with not as advanced proprioception and hand-eye coordination. 

These balls are typically used on a mini-court or red ball court which is played across the court with a miniature net