anaximander's math was great however if you are planning to use fat quarters you will only get 12 5.5" squares out of each fat quarter if you are really lucky. The fat quarter should be approximately 22"x18" and 22" is exactly what you need to get 4 blocks across. In reality some of that 22" might not be usable because of the printing or lack of printing on the selvage edge or because the fabric wasn't quite 44" wide or was cut slightly crooked.
Unfortunately that means you will probably need to purchase 12 fat quarters for each quilt top. Also you will have a lot of "waste". If you cut rectangles instead of squares you would get better use of your fat quarters. 5"x4" cut (4.5"x3.5 sewn) you could get 16 rectangles out of each fat. You would still need 10 fats for each quilt: 4.5"x10=45" wide and 3.5"x16=56" long. Incidentally - this same rectangle thing could be done with regular quarter yards and unless the fats are on sale the regular quarter yards will be less expensive.
There are some very good patterns/books that are specifically for fat quarters. You might look around while at the store. Make sure you look at the cutting diagram though - some "fat quarter" quilt patterns are labeled that just because they can be made using fat quarter but they don't really make good use of the fat.
No affiliation but two of my favorites are Yellow Brick Road by Atkinson Designs (or any of her booklets) and Turning Twenty - they are both quick, easy, and use just about as much of the fat quarter as you can. Yellow Brick road has smaller "pieces" while the Turning Twenty has "big pieces" so depending on the fabric one might be better than the other. While the Turning Twenty is bigger than you need (70"x86") you could probably make blocks for 1 TT quilt then use half the blocks in one quilt and the other half for the second. Add borders and you could easily get them to the size you need. Using different borders and backing would let each brother know which one was theirs. The Yellow Brick Road "twin size" uses 12 fats and is bigger than you want but if you leave off the boarder it would make about the size you want.
I've not actually seen the pattern but it looks like Cheaper by the Dozen and Cheaper by the Half Dozen look like they use the fat quarter efficiently. It is based on simple rectangles that are sewn together to make squares. It does have borders but to make it without borders you could change the dimensions of the rectangles I mentioned above. Cut them 5"x3.5" (It looks likethe actual rectangles in that pattern are larger - maybe 6"x9" but then if your fat quarter isn't cut perfect you could have a problem) you would get 20 from each fat. Sew them into "blocks/squares" by sewing 3 together along the log side then sew two of these together along the short ends and you would end up with 9.5" blocks. The advantage of this is you could then rotate the blocks. Google "Cheaper by the Dozen" to see what I mean. Of course for a couple of boys maybe the "design" aspect of the quilt isn't as important
Sorry this is so long. Hope it helps. It has certainly helped me avoid doing something I ought to be doing and instead doing something I love doing