My husband built a booster/high chair for our little girls a few years back that is a lot like the child seats in many restaurants, except to allow it to fit an older preschooler, ours doesn’t have the front bar. When our little grandsons got big enough to try it out, it was obvious they needed a bit of help staying in. Here’s how I made a strap for our seat. Click on any of the pictures to enlarge them. I think something similar to this would work to belt a child into a full-size dining room chair.
I began with two kinds of fabric and some quilt batting. The first thing I made was a long belt, big enough to wrap from the front around to the back of the high chair, and then around the baby. My belt is about 4 inches wide and about 40 inches long. I made the belt with two layers of quilt batting between two layers of fabric. I didn’t have any kind of pattern, so I did a lot of cutting, holding pieces up to the high chair, and figuring it things on the fly as I went. Your measurements will vary also, depending on the size of your chair and the way its rails are set up.
Next I cut and sewed a longish (20 inches?) narrow hourglass-shaped piece of fabric to be the seat piece. This again was two layers of fabric with two layers of batting sandwiched in the middle. I made it long enough to extend from the back of the baby’s waist (where it would be sewn to the belt), under his bum and up through his legs to his front waist. I also allowed enough length so that I could fold over the front edge to make a channel through which to thread the belt.
Here’s how it looks attached to the high chair, with the belt threaded through the front channel. The (unseen) snaps on the right side of the picture hold the belt to the high chair. The snaps on the left are the ones you snap after the baby goes into the chair.
Here’s a closeup of the snaps that hold the baby in. You can see that there are three different settings to allow for growth. I opted to have four snaps at each setting because I didn’t want them to pop apart with the little guys’ energetic movements.
Here’s how it looks when the snaps are done up.
And here’s little Ranger trying out the seat for the first time. Most of the time he’ll be sitting right up to the table, which makes it even less likely that he’ll go anywhere. But this adds a nice extra feeling of security, and keeps him from sliding too far down in the seat.