"Is this class easy?"
Semantically? In this case they both mean "easy". In other cases, yasashii carries meanings of "light, kind, relaxing" and kantan carries meanings of "simple, not complex or difficult"
やさしい is an adjective and かんたん is a noun. Both mean easy and both work in this sentence :)
Semantically they both mean the same here.
Syntactically, they also both take a copula in the polite form here. If it was informal, however, you could not put "da" after yasashii, only after kantan.
It's a bit complicated but Japanese doesn't have adjectives per se. "adjectives" that end in "i" act like verbs. Other "adjectives" are nouns that can modify things if you add "na"