Go to www.ancestry.com and use the SSDI search. Your John Sr will probably come up. There is a link there to generate a letter to send to Social Security. What they send you back will be his original application for Social Security number. Should give parents names. Social Security's fee for this is $27.Fee just went up from $7.
To look for his children, you can also check the SSDI to see if they are listed, (they will only come up if deceased).
Also on ancestry.com is a nationwide telephone directory, you can look for children on there.