Don't use full face masks for animal characters. Your children's voices won't project for singing or speaking lines, and they are also pretty uncomfortable usually! Use face paints ideally, or head bands with animal faces positioned well above the child's face.
Also, make sure that costumes generally aren't too long, heavy or cumbersome; nothing below ankle length. (I once witnessed an angel with the widest wings ever, who discovered that if he swung around very fast, he could knock other children off the edge of the stage!)