Something you may want to keep in mind when deciding on the block size is the finished size of the small squares in the center of the square-in-square block. With a 6" finished block (a 42x54 quilt) those little squares will be about 1.125". Making 10" blocks will give you a finished quilt roughly 70x90, still manageable for both you and the little guy, but increases the center squares to 1.75".
The yardage as needed for the pattern as drawn using 10" blocks would be roughly 2.875 yards of the black and 4.25 yards of the orange (plus backing and binding, of course). I know you already went shopping, sorry.
I like your pattern. Reminds me of a Brontosaurus or the Loch Ness Monster.