The Peacock iron is very popular for use by children. It is a variety of safety iron, designed so that the rubber ring is located on the outside of the foot, and will come away in the event of a fall. The idea of this is that the foot is released and prevents the rider from being "hung up".
These stirrups come complete with Rubber Rings and Leather Tabs. Both of these items are available separately here.
Additional grip may be offered through the use of rubber treads that can be fitted to the stirrup - available here.