Says WHO?

How do you explain to an emergent reader why the word WHO is actually pronounced HOO?

Santa laughs HO! HO! HO!  Mom exclaims "woo-hoo!" from time to time (when there is mozzarella cheese at shoprite?).  Dad works with the World Health Organisation - WHO.

It is causing MANY problems and tears at 23b.  Please, someone tell us:  Why is WHO not w-ho?