The 1940 federal census won't be released to the public until 2012, so your best bet would be some sort of local directory. But I'm not aware of any directories for Belfast or Waldo County published in that time period, much less one that would list the names of children.
The 1930 census shows the family with four daughters.
I checked Ancestry for city directories of Belfast, but the latest they have cataloged is 1913. As Chris said, the children's names aren't listed; but the head of household is listed, with occupation and address.