Wired computer mice might seem archaic, but they're incredibly easy to use. Children, or adults with smaller hands, might find that this mouse works well for them.
From Amazon
BenQ Zowie EC3-C Ergonomic eSports Gaming Mouse | Shorter Overall Length for More Flexibility in Vertical Movement & Flick | Paracord Cable & 24-step Scroll Wheel | Matte Black Coating | Small Size
Cazaar Fancy Dress
Zowie - EC3-C Mouse for Esports - Small