If Tam had been Janet's playmate as a child, this absolutely implies that he was considered the legitimate and baptized son of a noble family. He may have been a changeling, though, son of an elf knight and lady. Or he may just have lied ...

Folk tales are usually about powerful persons, including wizards etc., to emphasize the need to make up one's mind and take responsibility for one's decisions. Ordinary people often feel that they have little to decide at all, but if the situation occurs, they must be prepared, as if they were queens or fairies. For example, foolish wishes are a motif known to the whole world, in folk tales and reality; we must be warned that they may come true!