Stitch in the ditch and then make an x across the big blocks. Can "keep on going" across the quilt with making the diagonal stitching.

That's what I would do. Keep it simple. That way those luscious fabrics can serve as an "I Spy."

You could use simple pictures out of a kids coloring book as the patterns for Quilting in each square like suggested above...

I am usually a minimalist with my quilting except with a child's quilt. I figure it will get dragged around and washed a lot. Because of this, I do quite a bit of quilting. With this quilt I would SID the sashing and perhaps do a small center square in each block. I would then do diagonal stitching thru the cornerstones across the blocks all the way. I think this would keep it safe no matter what the child did with it. Hopefully they will have it forever (my kids took there little kid quilts with them when they moved out!) Great quilt- so happy! I think the yellow cornerstone are the perfect balancing touch.
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What a darling quilt!

I agree with YC quilter, I would do straight, or even meandering lines corner to corner through the blocks, just keep it simple.

Do you and OP have a pattern for this or did you just make your own design? I found some fabric in my scraps that would look cute in this block, but I always seem to need a pattern:)

Depends what look you want and how much trouble you are willing to go to. I like the suggestions everyone has. I like the idea of doing something different in each block, but would keep it simple.

Yep, I would go with the animal in each square or whatever fabric it is. :)

I would do an allover meander. This is probably going to be a child's quilt that will be loved and used until it falls apart.