Today's monofilament (both nylon and polyester) threads are not the "fishing line" of a decade ago. Some are as fine as a human hair and yes virtually transparent. I showed my monofilament to a client who used to be a surgical nurse and she said it looked like the suturing material they used when they had to do surgery on eyes! So that should give you a good indication of how fine it is and it is not stiff and pokey anymore either. Another option for thread is something super fine like silk (very expensive) or Invisifil polyester by Wonderfil threads. Both are 100 weight and sink right down into the fabric and batting leaving nothing but texture behind. They tend to blend in and reflect whatever color they are laying on so if you used a soft gray it should mix right in with every color on the quilt.
Because this quilt is for a 5 year old and has all the potential of becoming a favorite and "roughly" loved quilt I would opt for an all over meander that is fairly dense. It will help the quilt survive all the inevitable washings and it won't detract from the design at all.