The Doctor's Lady episode: Doctor Baker falls in love wih Harriet's niece, Kate. He has to admit that she is too young for him when he sees Kate playing with Mary and Laura.
What about the "miracle" episodes: Laura up on the mountain with Ernest Borgnine seems to be an hermit but really is an "angel". Adam regaining his site with a knock on the head. When Charles refuses to admit that James will likely never recover from his gunshot wound, he leaves Walnut Grove with his adopted son. He builds an altar in the woods and hopes that God will bring a miracle.