Thanks for the nice comments and fastening suggestions- I'm pleased to say the off-cut coat is finished and C seems to like wearing it (although the hood is very snug).
I managed to pluck up the courage to use the toggle buttons I'd bought for a traditional looking duffle coat fastening. The buttons don't actually have holes through them for a cord, just a groove around the middle. I hitched some hair elastics (the metal free ones, thanks for the idea Sooz) around the groove and pulled really tight to nestle into the groove, then stitched a triangle of leather over the elastic loop end. In the process I finally got around to using the leather sewing machine needles I bought ages ago, in conjunction with some black top stitching thread, and was pleased to discover that sewing leather onto layers of wool and lining was easier than I thought it would be. I used the hair elastics for the fixing loops too, and then to keep the buttons in position I did some hand stitching over the elastic very close to the button. Close up pic of the fastenings here. This (I think) makes it easier to do the fastenings up and means the fronts don't move around as much, eliminating the need for the snaps or secondary fastenings behind the toggles. It seemed to work fine today when we undertook a family excursion to the Melbourne Museum.
It was a delight to see him running around in it, and aside from the hood, it should see us through the next year (I'm contemplating taking the hood off and trying to insert an extra piece to give him some more head room, if I still have enough scraps). Another real pleasure of the coat was the joy of working with good quality wool fabrics. They were so easy to cut and sew- easy to ease the concave and convex curves together around the yoke, and a little steam work with the iron made everything sit satisfyingly well. A nice reminder as to why you should always buy the best quality fabric you can- it really does make the process easier and the result so much better.
More pics on Flickr here.