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if i sacrifice child of alara as part of dread returns flashback cost does childs ability go on the stack and resolve before dread return resolves or does child wait until dread return resolves to go on the stack?

for example say i have child of alara, wall of omens and wood elves in play and i cast dread return from my graveyard to bring back a magister sphinx from my graveyard.  will child wipe the board before the sphinx come into play or will dread return resolve bringing the sphinx back and then child or alaras ability goes on the stack and kills my sphinx??