The enclosure must be as large as possible and both predator proof and escape proof. This means that it must have a screen cover of hardware cloth or chicken wire. It only takes a second for a skunk or a raccoon to hop over a low perimeter wall, snatch a baby and hop out again. You must not allow the opportunity for this to happen. There should be more fencing buried in a trench on the inside perimeter of the wall to discourage digging carnivores and escapes. The enclosure should be sited to receive a southern exposure, after all letting them get some sun is a major reason for putting them outdoors. It should also be sited in a location that has not been exposed to herbicides or pesticides for at least one and hopefully two years, the reason for this is self explanatory.
The enclosure should be heavily planted with clover, dandelion, plantain and grasses. If you are not sure that a weed is edible, root it out. The Turtle List Serve is a good place to get advice and find web links to sites of edible plants for your tortoises. The hatchlings and juveniles will need a hide box though they may prefer to dig their own "pad", "scrape" or "resting form". The hide box should be sited in the coolest spot in the enclosure, generally the northeast corner. There must also be a shallow source of water available at all times. Some boulders or logs may be added to break up sight lines and to make for a more natural appearance.
The tortoise should be examined daily just as if it were being kept inside. Any loss of weight greater than 10% or strange swelling should be investigated immediately. Supplemental feedings may not be necessary depending on species, but should be provided on a reduced schedule 3 or 4 times a week as a vehicle for giving calcium supplements to the tortoise. Just as indoors a cuttlebone should be provided at all times. Obviously tortoises that have diets not met by the flora growing in the enclosure should receive supplementation.
If you are certain that the pen is predator proof, escape proof, the hatchling is in fine health AND that the temperature will not drop below 60 degrees at night they may be left outdoors. As stated earlier I prefer not to take the chance.