A good starting point for any Greek god costume is a white sheet wrapped like a toga and a pair of sandals. With this basic costume assembled, you can add accessories that symbolise specific gods, like wings, arrows or crowns. Customise a basic kid's toga and sandals costume to suit any Greek deity.
Zeus is the king of the Greek gods. He is usually depicted bearded and holding either a sceptre or a lightning bolt. Take the basic toga and sandals costume and add a gold crown to symbolise royalty. Make a sceptre or lightning bolt out of cardboard or wood painted gold. You can either paint on a beard or make one by gluing cotton balls to construction paper and tying it around the child's face with string.
Hermes is the messenger to the gods and the god of travel. He is known for having winged sandals. Attach cardboard wings to the sides of your child's sandals with string or twist-ties. You could also make or buy wings for your child to wear on his back. Cut cardboard into wings and cover it in gold glitter or feathers. Use arm loops made from string or plastic to keep the wings on your child's back.
Poseidon is the god of the sea. His trademark is his trident. You can make one out of sturdy cardboard and paint it gold or cover it in gold glitter. Poseidon is often depicted with long hair and a crown. You could buy a wig and a crown or make a crown out of cardboard. Incorporate seashell designs into the crown or add a seashell necklace. Poseidon is sometimes depicted as a merman, so painting fish scales on your child's face or arms or even incorporating a mermaid-like tail could also work.
Dionysus is the god of the arts, particularly theatre. He is known for enjoying food and festivities. Add a gold grapevine crown and a gold drama or comedy mask to the basic toga outfit. You can create a theatre mask for your child to wear by cutting out eye holes and either a smiling or frowning mouth from a paper plate. Paint the paper plate gold and attach an elasticated string to go around the back of the head. You could also have your child hold bunches of grapes or copies of ancient plays by writers like Aristophanes, Euripides or Sophocles.