Because there is no hard numbers as we can really do is give you some general rules to watch out for.

In this case, the main thing to consider is the time you lose for a having a child which owns a large hunk of memory versus having to reload that child every once in a while. If the child slows down the system(example, because of a paging file), then you should reduce the MaxRequestsPerChild per child until that stops being an issue.