When can they go outside?

[friendlyearth]

Our three Indian Runners are now 11 days, 18 days, and 4 weeks old. When can they stay outside? Evening temperatures are still in the mid-40's. Will a heat lamp keep them warm enough in their box outside? We are leaving them outside during the day and bringing them in at night, but they are getting too big for the inside container and doing nothing but making a big mess.

Thanks!