Buy leather shoes. Leather ballet shoes will stretch much better than canvas ones.
Loosen or remove the drawstring. This should provide a bit more room for the whole foot.
Have your child walk around the house in them. While ballet shoes should never be worn on the street, extra walking in the house can help stretch them out.
Put a wooden spoon or dowel in them to stretch them. Either use the spoon to gently push the toe forward or place the shoe around the spoon.