Okay here it is finally! Sorry it took so long! Alright materials... old t shirt and a t-shirt that fits your child.
Lay your t-shirt that fits over the t-shirt you want to resize. In this case I am keeping the collar and shoulder seams on my resized shirt because they are small enough. **I will have the option if your collar is too big, how to add a rib knit at the end** If your collar is fine, cut around the t-shirt arms and bottom leaving yourself a seam allowance! (If you look at your shirt you will notice they hem under about an inch and a half on the bottom so leave a bigger seam allowance on the bottom).
Next you will lay your sleeve over your shirt you are resizing and cut around that as well. At Nancy's Couture she cuts her sleeves off from the bottom...mine are cut from the top (aka top is open).
Now here is where I did things different from everyone else. I sewed both sides of my shirt. Then on my now smaller sleeves, I sewed closed the top so it is a tube again (just like you would do if you had a pattern) - **right sides together**
So now both my side seams are re-sewn and my sleeves are re-sewn other than where they attach to the shirt. Now I will attach my sleeves to the shirt. To do this, turn your tshirt and your sleeves right side out again - like you are looking at your shirt normally. Basically you will kind of need to insert your tshirt into your sleeve a little to get the right sides of your shoulders seams and bottom seams to match up to the corners of your shirt where you attach your sleeves (this is really a lot harder to explain than I thought-which is why I take a lot of pics :) Also I cut too little of my sleeve so I had a lot more sleeve width, so it didn't match up to my shirt well, so I just sewed another seam to make it smaller on top and then re-pinned.
If you collar was too big in the beginning you need to cut around your WHOLE shirt including the shoulders and collar (leaving seam allowance)(sorry for the pics of 2 different shirts in this part)
I got my rib knit at Joanns, it takes a very small amount for me an 1/8 yard could make a few shirts. Measure around the front and back of your collar of the shirt..mine was 14 inches, so I cut my rib knit at 12 inches length (because it stretches)and 2 and 1/4 width (for seam allowance and fold, I wanted a 1 inch collar). There is a formula for this, using 2/3rds..but I just guesstimated. Also you want to cut so the ribbing is horizontal when you use it which allows it to stretch the correct way once it is on your collar.
Before you start you need to sew one shoulder seam closed (rs together) so that you can sew on your collar as one continuous piece. (I didn't take a pic here so if you need one, let me know) Here you can see I folded over my ribbing. Now I am going to scoot that raw edge of my rib knit up to that raw edge of the tshirt and sew (right sides together) When you sew you are supposed to stretch the rib knit and sew. The FIRST time I did this I stretched WAY too much and it was like pleats on the front of my shirt and the collar was really small. Cue seam ripper. So basically its a very slight stretch.
When you finish you might have some overhang..that is fine because now you are going to sew that other shoulder seam closed and enclose your collar and ribbing (rs together)