My symapthy and prayers on the loss of your grandson...It's hard.
Many have offered hints for making t-shirt quilts, which are fun to make. My choice for interfacing is either woven or Easy-knit (both are iron-on interfacings). I used Pellon in my last t-shirt quilt, and would only do it again in a pinch, because eventually it starts letting loose, even though I did use a press sheet...I'll be doing FMG in the blocks themselves to secure everything.