Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add peas, cover, and steam until tender, about 15 minutes.
Place peas and breast milk in a blender or food processor; puree until creamy. Strain mixture through a fine-mesh sieve to remove any lumps.
Spread pea mixture into an ice cube tray; cover with plastic wrap and freeze until solid, 2 to 3 hours. Transfer frozen cubes to a resealable plastic bag and store in the freezer.
To serve, heat cubes in the microwave for about 30 seconds until warm but not hot. Stir thoroughly.