Squash Blossom is a good choice. It is a 2 color block on a neutral background which could be grey and Hawaiian. Then it has a border and some sashing. You will have to add the border and sashing no matter what pattern unless you just do grey blocks. The squash blossom would have the grey shirt material cut into 3 1/2 squares so you could get squares from the sleeves as well as from the front and back. Each block would have 2 HST of the Hawaiian shirts. The borders and sashings could be selected to emphasis the colors in the HST. This way the quilt might not end up looking totally bland and blah but have some personality and I'm sure they remember their dad as a personable guy. Good luck on whatever you choose.