When?
Whenever they want to, but not when:

• You decide it's too cold
  We've found that bare feet are OK almost down to freezing point as long as the core body temperature is fine. If a child is walking around and not complaining, he/she is probably fine. The thing to understand is that our extremities are designed reduce heat loss by dropping to a cooler temperature than our core. This coolness is not, of itself, a problem. The belief that children's feet must be kept at a constant (and rather sweaty) 98 degrees has no basis in science.

• You decide it's too hot
  If you know the sidewalks are sizzling or you see them jumping around, suggest they put their sandals on. Also teach them the techniques for coping with this.

• You decide it's too hazardous
  As an example, areas that simply have too much broken glass. These are generally pretty rare. However this is a case where we would insist on shoes, especially for younger children since they aren't quite as able to adapt to such hazards as older children are. It's a judgment call ... but it's always best to explain to your children the reasoning behind your judgment.

• You don't want to cope with hassle from "shoe-police"
  This is a tough one. We don't want our children to get the impression that stores and restaurants have arbitrary power over the way we dress (they don't ... we can just take our business elsewhere). We should teach our children that we are the customers and that we feel that such policies are silly. However, sometimes we are in a rush and it's best to just play along for convenience sake. What is surprising to most people, is that this hassle actually occurs quite rarely.