This was close to my first thought as well. Grey thread would blend top and bottom. Simply quilt an X through the big squares and maybe a sepentine line through the sashing? Or do bubbles in the squares. But not monofilament. I know you are not feeling brave , right? But trust yourself to quilt a simple motif like X's or bubbles or squares in the squares. You'll like it more than you think. Monofilament, if it comes loose, can snag a child's toe and hurt like heck (or do damage). Plus it's a pain to work with (or can be) and not comfortable against the skin.

Someone on this board turned me on to Superior invisible thread when I asked about it. I have used nylon in the past and hated it. The Superior sews just like other polyester threads EXCEPT you have to loosen your top tension way down so you don't get loopies on the back of your quilt. I don't know how to adjust the tension if you use the Superior polyester in top and bobbin. Experiment with fabric scraps and left over batting.

Remember that you need to measure in all directions, including diagonally across the square. If you used charm squares, they measure 4.5 in across, but over 6in diagonally.

I would tend to shy away from monofilament thread in a child's quilt (it can also be a bit tricky at first try). What you might try is using a matching thread in the bobbin and a blue on the top that matches your cornerstones. If you are not sure about your straight lines, then you could do a stitch in the ditch in the cornerstone and then 'wobble in' in the square. Sort of a small orange peel/crescent moon look. This gentle curve would be easy for a beginner and forgiving as in not showing little imperfections to a straight line. Because your batting says 10" is acceptable, you could just do all the lines either vertical or horizontal. If you felt good about it, then do both. But this would give you on out. The main thing is to do something you are comfortable with. If you make it very difficult and it stops being enjoyable, it will become a work project and destined to become a UFO!

I would consider quilting 1/4" outside the blocks rather than stitch in the ditch. If you are new to quilting this might be easier and also using a bright multicolor thread would be a nice accent around each block.
This is a sweet I Spy quilt.
peace

EDIT: Like bkay said is perfect.

I have used superior monopoly it doesn't disintegrate or poke as I used it in a garment washed hundreds of times

If you want the quilting to kind of recede into the background, you could always use a finer thread, such as Bottom Line. A lot of quilters use a 50 or 60 wt thread, Bottom Line is a 40 wt.

I actually really like when I can see the quilting on the back. :thumbup:

I quilted the one I made which is very like this, with a 1/4 inch from the edge square inside each square, and a serpentine stitch down the sashing, vertically and horizontally, and in the borders. I quilted Xs in the four cornerstones. I think my squares were five inches. As I recall, I used white thread (I made and gave it to a friend so can't look to see).

I made a blanket for one of my little nephews quilting roughly four inches apart (eyeballing it with a walking foot) and the Warm Co soft and bright poly batting and its five years old and going strong with frequent washing and drying, faded a titch but it's well-loved and well used. No worries :) my son has a quilt with six inch SITD done in a crosshatch and it's fine too

Im a fairly new quilter and I have a really hard time staying in the ditch and I get frustrated and critical of my work. I have started using a serpentine or zigzag stitch when I stitch in the ditch. I also do a lot of shadowing the ditch (more work) For color, I would use a color that blends for the bobbin and white or lt gray for the front (regular quilting thread). Keep it simple, you want your blocks to stand out not the quilting.