Try shifting the color wheel a bit on your colors- turn red to orangey-yellow that still maintains the warm color, give the yellow a shift to gold that will go well with the orangey-yellow but still not just blend, move the blue to a teal that goes well with the green. Then you almost have colors that are yellow and green in 4 different hues but just not quite, still a difference on the color wheel. It's very difficult to lose the primary colors because of their intensity, soft shades are probably not going to look as well with the kids' blocks. This doesn't sound like a quilt for pastel colors. It's too bad the mother's didn't just commission the quilt and then stay out of it.