Hopefully someone who really sews can weigh in but I have a general opinion about tools and art materials for kids, which is that parents should try to get the best they can afford. People take to things best when they have access to quality tools, be that art, music, sports...
I am a novice sewer, so you'll have to take my opinion with a grain of salt. I consider sewing machines to be safe for a 5 year old. She/he will need to be supervised but that will be the case with a child's machine as well, I imagine. I would just get a good, basic machine for your home.
Things I would look for:
*Ease of threading the needle and bobbin. And especially ease of getting the bobbin thread up (I thing this is the most stressful part of sewing).
*Good tension maintenance. I upgraded to a machine that is "better" (supposedly) but is known for tension issues. That's super frustrating.
Another thought would be to see if you can borrow a machine. Many people have a machine stored at home that they rarely use. Most folks with a machine wouldn't miss it if it was gone for a month or two or six. ;-) Maybe you could borrow one to see if your DC is really ready and/or discover on your own what sorts of characteristics are important to your child as a sewer.
Troll? Here's me...