If you want full freedom of movement, you should opt for a wireless mouse such as this one. Children, or adults with smaller hands, might find that this mouse works well for them.
From Amazon
Adesso 2.4Ghz Wireless Programmable Ambidextrous Ergonomic Trackball Mouse, with, Black, iMouse T40
Memory C
Adesso iMouse T40 Wireless Trackball Optical Mouse