In The Woods of Waiting. The Volkdrow Chronicles, #0.5

Brenna, Franklin, and Jerm are three normal children in Red Fern. Well. they're as 'normal' as things get in Red Fern. They live in Ickne Noir's Home for Lost Children, but things are not quite as they seem. You see, Ickne Noir's may be a children's home, but it's also the place where neighbouring realms send their young spies and assassins. All the children there are deadly. Apart from Frankin. but you'll find out more about him soon enough. A beautiful song leads them all into the menacing and mysterious Woods of Waiting.
That song will set all three towards fates they could never imagine. Dark paths and dangerous things await in the Woods of Waiting. Caution: This fiendish fantasy may contain monsters and swearing. Actually, it absolutely does contain both of those things. Prequel to The Volkdrow Chronicles.