This took me 5 min. Its an Elco, NV HD shirt in XL that fit, but barely came down to my belt. (why the fuck is every shirt that short? Im not tall, only 5 10, fuck...)
Turn it inside out, and use a piece of chalk and trace the outline of your kids shirt, or if you want to make a slim fit shirt for your Ol' lady or something, just use a shirt that fits who ever the way they like it....
then just sew it right from the bottom up to the arm pit, set your needle and spin it, and sew right across and off of the sleeve!
Since I was making a XL shirt fit a 4 year old girl, I then marked it where it would fit her like a normal shirt, then turned it inside out on the bottom, and sewed around the bottom. Then you just take the scissors and cut off all the flab. Turn it back right side out, and BAM! Cool t shirts are now made for little kids too!
you can get a sewing machine that will do regular t shirt material used for like 30 bucks on CL. A while ago I restored a 1938 Singer cabinet machine and it will do cloths all day long.