The trouble of ChildGetCount is that in U++, it is "loop" function (you have to go through the list of child and count them) - you probably do not want to encourage users to depend on such method, you better to force them to organize things differently.
As for HeaderCtrl::GetCount, it is different beast altogether - HeaderCtrl column tabs are not Ctrls.
However, you might be right about Splitter. Therefore, it now has "GetCount", which returns the number of split-panes.