A land controlled by a weird cult with priests having magical powers. It has an underground resistance. The heroes might break some rule accidentally or just look suspicious. Or maybe they appear in a wrong place at a wrong time and learn some secret. They become involved in this struggle, one side is trying to help them, the other side - kill them.
A land inhabited by intelligent creatures who have never seen a human (dragons, centaurs, ents, golems, etc.) These creatures find kids amusing and keep them as pets. Kids must escape or convince the creatures to let them go.
A land cursed with a powerful and strange spell (like everyone who says "peanuts" starts speaking backwards). One of the kids becomes affected. They need to break the spell or find a loophole in it before they can continue.