>I have a tool rental system for which I would like to avoid saving the main issue ticket until I am sure that I have at least one valid detail row. Of course, I don't want to save the detail row without a valid ticket either. I thought I could do that by making the detail row a child of the ticket but the parent rules are validated before checking the child records. Is this possible? If so, where do I put the check for existing child records? Thanks.

I would consider doing as you indicated and creating a parent entity, then a child entity. When the user completes the child entity, you can call any rules on it yourself prior to saving the whole thing. Then if the child record is valid, call the save on the parent which would also save the child. If they never complete the child, then don't save anything.
Timothy