An edit control CAN be used in a child dialog. I suspect you have another problem and that why it's not working. I'd suggest posting relevant code with your question and maybe people may be able to spot your actual problem.
You're probably experiencing this issue:
http://support.microsoft.com/kb/230587[
^]
Basically, the child window that is the parent to your edit control is intercepting the messages that the edit control needs in order to be used. You can change this behavior, but it's probably easier to use a different style instead of changing the behavior of WS_CHILD.