>>I just re-read your original question again. You don't need to group by ChildPK as in your example (unless it was a complete mock) you seemed to take the first record per Parent Key, not by Parent and ChildKey combined.
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>When I have more than one child record combinations, I have to take the one with the highest value in the child field. So far, this would work as we would only have two records in a combination, at most, and the one with the highest value would work.

You don't want to GROUP by ChildKey.