Eye contact is crucial in attunement and ultimately the attachment process. Some children with Reactive Attachment disorder have difficulty maintaining eye contact. I believe that eye contact does one of two things to these children. One, it can be perceived as threatening to some children with RAD. Second, it elicits feelings of shame in these children as the "self" has been exposed.
While I would not encourage a caregiver to force eye contact, I would encourage the caregiver to have the child practice eye contact. It is important to accept where these children are at. If a child cannot maintain any type of eye contact, I would get a knitted-blanket with holes in it. Place the blanket over the child's head. Thus, the child can look at you but does not have to be exposed to your eye contact. Then just have a short conversation with the child. Do this several times a day.
If the child is able to tolerate limited amounts of eye contact, you can have the child look into your eyes periodically throughout the day while having a conversation. Have the child maintain eye contact for as long as he or she is able to tolerate it.