"'But before the boy knows enough to reject the wrong and choose the right, the land of the two kings you dread will be laid waste.'" NIV translation
Isaiah continues by telling Ahaz that the kings that are coming against Jerusalem will fail. This was prophesied to occur before Jesus was old enough to choose right and it did happen.
"'The Lord will bring on you and on your people and on the house of your father a time unlike any since Ephraim broke away from Judah - he will bring the king of Assyria.'" NIV translation
Isaiah now turns to the coming judgment on Ahaz and the people. He tells of the coming invasion of the Assyrians and the fall of Jerusalem.
"In that day the Lord will whistle for flies from the distant streams of Egypt and for bees from the land of Assyria. They will all come and settle in the steep ravines and in the crevices in the rocks, on all the thornbushes and at all the water holes." NIV translation
God is bringing judgment on His people and we see that He is going to use the nations around them. Flies are a bother but bees actually cause pain and so Egypt will be a nuisance but the actual pain will be from the Assyrians.
The use of insects to describe this coming judgment reminded them (and us) of the plagues that God brought on Egypt to get them to let His people go (see Exodus chapters 7 through 10). God is telling them that these nations will overtake them and be everywhere in their land.
"In that day the Lord will use a razor hired from beyond the River - the king of Assyria - to shave your head and the hair of your legs, and to take off your beards also." NIV translation
The river that is spoken of is the Euphrates River which runs through the modern country of Iraq. The Assyrians were feared and they were known for their brutality toward their enemies. The shaving of the head and beards was a means of humiliating a defeated enemy.