Fix test suite for children

Copied the test for statistics test, which is the most comprehensive
test written thus far and added yet another case. Found one case where
the test suite tested for incorrect results, which I fixed.

Also add the one extra case to the statistics test suite for
completeness.

Part of #518.
Merge pull request #545 from plajjan/518-children-calc

Fix test suite for children

Fixes #518