Should a toddler's arm fracture be in a cast?

Yes: Must of the time yes, however depend upon where the fx is, the particular bone involved and the characteristic of the fx.It varies from a simple splint to a full arm cast.

Depends: The fracture label can be applied to a torus fracture, which is like a dent in a bear can & doesn't have any instability at all. If anything these are sometimes put in a splint more to remind the kid s/he is injured until it heals. Casting may be delayed until swelling subsides, so it can work better. It's primary role is providing a stable protected positioning. Not all fractures need it.