To tone down the sauce for little mouths, omit some of the chiles.
Cut chiles in half lengthwise; remove and discard seeds and any ribs. Set aside. In a small saucepan, bring chicken stock and the water to a simmer. Add chiles; simmer until tender, about 15 minutes. Set aside. Fill a medium saucepan with water; bring to a boil. Fill a large bowl with ice and water; set aside. Cut an X in tomato bottoms; add to boiling water one at a time. Simmer until skin begins to peel. Transfer tomatoes to ice bath immediately; cool. When cool, peel and discard skins; chop, and set aside.
Place poblano pepper directly onto the trivet of a gas stove over high heat. Turn pepper with tongs as each side blackens. (You may also place peppers on a baking pan and broil in the oven, turning as each side becomes charred.) Transfer to a bowl, cover with plastic wrap, and let steam for about 15 minutes. Peel off blackened skin; discard. Cut pepper in half; discard seeds. Roughly chop pepper, and set aside along with any juices that have collected in the bowl.
Heat olive oil in a large high-sided skillet over medium heat. Add onion, scallions, and garlic; saute until translucent, about 10 minutes. Add tomatoes, tomato paste, red chiles and liquid, poblanos and liquid, cumin, oregano, and salt. Stir to combine. Simmer gently for 15 minutes. Transfer to the jar of a blender or a food processor; puree until smooth. Return to pan; keep hot if assembling enchiladas, or refrigerate up to 4 days.