So many possibilities. Personally, I'd ask each kid/grandkid to pick a shape for me to quilt in their block (from a list). To add unity to the quilt, I'd pick the shapes based on what I see in some of the drawings (hearts, clouds, circles, arches, 6-pointed stars & butterflies). I'd either use a thin white cotton thread or a clear poly thread for quilting -- or maybe a very, very light grey. Depending on the drawing/shape, I'd either echo the shape or repeat the shape.
Other possibilities:
- straight line: quilt a 9-patch on point in those squares
- custom FMQ based on drawing: outline each drawing as much as possible & then use filler around it if needed (for the bottom right, try to group the scribbles into shapes as much as possible -- may need some input from the child)
- thread painting: change out your thread to match the color of each part of the drawing & outline it in the matching color; then fill with white or clear thread