Take away the 1/2 cup. When the recipe calls for half cup, discuss how you could use two 1/4 cups to equal 1/2. You could 'prove' it to the child by pouring two 1/4 cups into the 1/2 cup.
Help the child measure; omit asking the child what 1/2 and 1/4 means. Adjust the recipe to use only 1 cup and 1/2 cup.