They are smashing windows because the WTO is refraining from interfering in the sovereign right of nations to make decisions like this for themselves. But schooling everybody is something only fairly rich countries can afford to do. Great Britain did not deliver universal elementary education until the 1890s. The United States did not do it until the 1920s. Canada did not do it until the 1950s.
Child labour is caused by poverty.
When South Korea was as poor as India, it too was disgraced by child labour. When India becomes as rich as South Korea, its problem of child labour will vanish as entirely as South Korea's has done. But the only way India can grow as rich as South Korea is by trade. Trying to banish child labour by refusing to trade with countries where it exists is like trying to halt tuberculosis by refusing to sell vaccine to countries whose people are perverse enough to get the disease.