Last summer my grandkids, then 3 and 5, helped make their own pillowcases using my Brother cs6000i machine, which is an inexpensive, very easy to use machine that has very good reviews. What I like about this machine is it has both speed control and the option of start/stop button on the machine rather than the pedal. I didn't have to start them with learning to control their foot and speed while also trying to control their hands and fabric. I had the pillowcases all ready cut out and in the "tube" (used Missouri Star video instructions) and pinned the seams. The kids were on my lap and helped with the sewing, mostly learning to start/stop with the button every time we came to pin. They removed the pin. I warned them enough about the needle that they were afraid of it. It was a great first project, they didn't know what they were making and had fun turning the tube inside out. Their attention spans were short so they took turns at the machine. A few months later they moved 800 miles away. They are proud of their pillowcases and remember making them with me. Last month ago we went to visit and I took my machine. They helped make simple valances for their bedroom windows.
My daughter's Brother machine does not have the option of the button start/stop, she has to use the pedal. She wishes she had that feature for the kids. Another family member got the now 6 yr old a toy Singer machine for Christmas. She was so excited, telling me all about it, but it's just been a frustration for them and a big disappointment. I wouldn't start a child out with a toy.