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Only a thought, not a fully formed answer....Create a before save calculated attribute that counts the number of children and the number of parents. Then use this on the criteria. OK existing incidents might need some carefuly handling, but new ones should be OK. The criteria for something being a child would then be that the count of parents was > 0 and similar for being a parent.
Ideally this sort of calculation would be possible in a query too. There is an ER for this which also contains other suggestions, so feel free to vote and add more suggestions for improving the query designer
Thanks for that Dave, I will give it a go. Voted on your ER ;-)