Venom is one of the most popular villains of the Spider-Man fans and it's no surprise that kids want to dress up like him. Contrary to the comic books' hero though, his commercially sold costumes are few and rare to find. This shouldn't discourage his fans, as creating a mask resembling Venom's face, using easy to find materials, is not as hard as it seems.
Cut the mouth hole of the balaclava to resemble the sinister smile of Venom. Don't reach the ears; a smile slightly bigger than the eyes' distance is enough.
Draw Venom's eyes on your old T-shirt with a marker. Consult a comic book or the Internet to ensure it is the same shape. Carefully cut the eyes from the shirt.
Cut the Venom's mouth shape from a white carton. Paint the gum areas red.
Create the Venom's tongue using carton again and paint it red at the end. Glue it to the inner part of the bottom teeth row.
Sew the mouth pieces to the balaclava from the inside.
Sew on the eyes.
Wear the mask. As the material of the eyes is thin and white you should be able to see, but not well enough to ensure the safety of you or your children, when wearing the mask. Open tiny holes with a pointed knife throughout the part where the initial balaclava eye holes were. This way you will be able to see clearly.