Isaac's birth grandfather, George Sjouke Riemersma, Joni's father, wrote a book chronicling their life. I am so glad he wrote it, as it will give Isaac an amazing look at who his grandfather was. Born in Holland, the Rimersmas moved to South Africa before Joni was born -- and that was where her life began. They were then sponsored and able to move to America. (Thus the reason Joni speaks both Dutch and English and also calls herself an African American.)
Here is the cover of the book and a link to a place you can read it online. Check out page 14, where Isaac's birth is recorded!