I found a whole bunch of drawings I made while I was 'writing' gum, trying to figure out what the grandpa would look like. I can't really remember, but apparently I wasn't really set on a grandfather and I explored the option of having a grandma instead. though now that I think about it, it may be that I was planning for a moment to have a cute old couple instead of just one grandparent. You can imagine why this went out the window real quick.
In any case, I got a whole bunch of pictures off the internet that I then drew from, trying to get a sense for what I wanted, let it brew for a while and finally came up with a face that I felt both interesting and that I was comfortable with.
The design then evolved a little as I was drawing the storyboard, but not very much. The kid on the other hand, I didn't even bother trying to design and left it for the time I would animate... Because honestly, the kid is not nearly as interesting as the grandpa.