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!