Have you tried pressing your seams open? It should help with the bulk in the center, be sure to shorten your stitch length if you do. Kimberly Jolly of the Fat Quarter Shop has a great video, (you can skip to 3:27) for how she deals with bulk. Also, if you are going to remake your corner triangles don’t be afraid to oversize them a bit for more wiggle room when trimming down, you’ve got this and either way it’s gonna be a treasured gift and a labor of love for your nephew.
https://youtu.be/aYLIVgDaChU