The babies would get half of their genetics from each parent - i.e. 25% SD, 25% Dwarf and the rest would be Mainland. Basically, they would get half the SD % and half the Dwarf % and half the Mainland % from each parent.
Mom is 50% SD and 50% Mainland = 25% SD and 25% Mainland passed onto offspring
Dad is 50% Dwarf and 50% Mainland = 25% Dwarf and 25% Mainland passed onto offspring
25% SD, 25% Dwarf, 50% Mainland =100%
Hopefully this helps!