Hello Tiffany - This was part of a recipe from my grandmother's notes for small infants - before they can eat solid foods. Your 18 month old can maybe eat seasoned mashed potatoes with garlic & onion, baked chicken with sage and tyme, and maybe some oatmeal with cinnamon, cloves and ginger. It only take a tiny, tiny amount of these herbs to be effective. Cloves, for example, is just 2.5mg daily per 30 pounds of body weight - this is .00125 of a teaspoon volume measurement(a TINY PINCH). He/she might be more willing to eat them in food than to swallow them in a liquid form.
My personal recommendation is still to keep Wormwood away from children - it's just too dangerous. The three year old can take Black-Walnut (15mg which is .005 teaspoon volume) - daily per 30 pounds of body weight. This and some of the above listed herbs that are great for parasite removal but do not carry the same dangers as Wormwood .