You announce Bone Splinters, thus putting it onto the stack. You pay its costs, and while doing so, you sacrifice Child of Alara, triggering his ability. When you finish paying the costs and making the choices, meaning, when the spell is ready to try and resolve, players would receive priority, so the trigger is put onto the stack, above Bone Splinters (thus, resolving first). The trigger resolves and nukes Reveillark, triggering HIS ability, allowing for his controller to return up to two target creatures with power 2 or less from his graveyard to play. After this resolves, Bone Splinters will try to resolve and will be countered upon resolution for having an illegal target (the now-deceased Reveillark).
Clear?