This may be the Spellkey series by Ann Downer-Hazell.
Summary of the first book, The Spellkey, from the entry for a trilogy compilation:
The heart of the story is a journey made by two outcasts: Caitlin, a wild girl-child accused of witchcraft and sentenced to exile in the Abbey of the Ninthstile; and The Badger, a disapointing ward of the Abbot of Thirdmoon See, who has been ordered to escort her. As they travel through the Thirteen Kingdoms, they are pursued and observed by a myterious man with red hair who seems to defeat them at every turn and by Caitlin's own dark visions, which haunt her with the fearsome destiny they predict. The answer to it all seems to be Spellkey; finding it and solving its Mystery may mean destruction for the cruel rulers of the Pentacle and a solution for Caitlin an The Badger, who need one desperately since they are falling in love...
Summary of the second book, The Glass Salamander:
Caitlin's training as a seer on the isle Chameol is interrupted by the return of the evil necromancer Myrrhlock, while her lost lover the Badger investigates an outbreak of madness in two of the lower kingdoms. Sequel to "The Spellkey".
And summary of the third book, The Books of the Keepers:
Bram, kidnapped to an underground kingdom to be consort to the Elf Queen, escapes with the help of the Goblin Pretender and returns to his mother Caitlin, who is seeking the Books of ancient magic from the Elder Age. Sequel to The Spellkey and The Glass Salamander..
I found the book by searching for book series fairy badger world changeling child, which brought up The Glass Salamander among the results.