To sign allow, take both hands and form open, flat hands facing each other, all fingers held together on each hand, but at diagonal angles so that they aren't parallel, more like angled inward towards your torso at 45 degrees. Sweep both open hands in a curved fashion slightly upward and outward.
The allow sign is signed almost like street, but in a more angled, curved fashion (not parallel and straight, like the latter sign).
Teach your toddler the allow sign when you are giving them explicit permission to do something.
Teach your toddler this sign as they grow aware of certain things or activities that require parental permission. Also, in the context of their relations with siblings, friends, or daycare classmates, they can learn which actions are permissible.