Not sure what to make of data suggesting another child. On Geni, I'm not seeing a Thomas “Joanes” born in Dorchester (as shown below) with a father named “Isack Joanes”.
Name: Thomas Joanes
Sex: Male
Father's Name: Isack Joanes
Father's Sex: Male
Event Type: Christening
Event Date: 18 Mar 1659 [perhaps 1660]
Event Place: Dorchester, Suffolk, Massachusetts Bay, British Colonial America
"Massachusetts Births and Christenings, 1639-1915", https://familysearch.org/ark:/61903/1:1:FZDC-LNL
The image to this data is has not been linked, but is apparently among 300 document images here: https://www.familysearch.org/ark:/61903/3:1:3Q9M-CSM7-LHMD?mode=g
