The "where is this child" query is unaffected by the ordering, whether Z-order, Hilbert, or other. However querying "which children are in this area" requires that you come up with corresponding ranges along the curve. This is where the Hilbert curve is slightly better because in many cases the same area can be covered by fewer ranges.

Follow up questions: How do the number of ranges compare with the different orderings? How much does having fewer range segments affect database query performance? Does it make up for the added computational complexity of Hilbert curves? I've not answers, but these can be answered by science.