Oh no! Thank goodness you washed it before gifting it. I am really surprised at how many seams gave way on you. I am thinking the fabric frayed a lot but I am also thinking that when you joined the squares to the sashing strips perhaps the piece that was on the bottom wasn't all the way to the edge so you weren't catching a full quarter inch on one side. Are your squares flannel? They are also notorious for fraying.
It also appears that the SID you did wasn't enough as most of the seam splits appear to be in places where there is no quilting so the fabric and the seams couldn't handle the weight of the wet quilt in the washer. Even though so many battings say you can quilt up to 10" apart, it has been my experience that is not sufficent. I am so sorry this happened to you. I hate to say this but I think once you patch this you should probably add some more quilting so when your nephew gets it he doesn't find more popped seams after his first wash.