I had enough fabric left over from each of my Renfrews to make my kids their own tees too.
Logan's tee is made using the Vintage V-neck pattern from Blank Slate Patterns. These are pdf patterns for boys and girls. There aren't a lot of cool little boy patterns available buy Blank Slate has a good selection. I made a 4T based on his measurements. There isn't a lot of room for growth so I will go up a size the next time.
Next up is the baby's tee. She just turned 1 year old and is about 20 pounds. I used Simplicity 1566. This is a good pattern for basic separates- tops, pants, jackets, vest, and jumper. I made a size large (18 months). Again, there's not a ton of room for growing. This was also super easy. The most time consuming (and annoying) part was hand sewing snaps on each shoulder seam. These are necessary though so that the top can fit over baby's head.
Both tees came together quickly and they were a great way to use up the leftover fabric.