An arm immobilizer prevents bending at the elbow. These are commercially available and cost around $40-$60. They can be used for many things - to prevent movement of an injured arm or prevent further damage to a surgery site. In this use, it is similar to the "Cone of Shame" or "Elizabethan collar" worn by pets to prevent them chewing the stitches. It is a commercial product that is easy to replicate in cheap materials.
One of my daughters had oral surgery and had the bad habit of sucking on her fingers - a bad combination because of damage to the stitches, higher risk of infection, etc. She had to wear "No-No" (brand name) for several weeks while it healed. This is where I first saw the idea. At some point, all we had to do was threaten her with "Do you need your NoNos?" and she would keep her hands out; she still wore them while she was sleeping.
Yesterday (holiday weekend) D1 pushed D3 down the steps. D3 spent four hours at the hospital emergency room getting her lip sewed back together. We even had them put a layer of super glue on top to protect the stitches. By the time we got home, D3 had pulled two of three stitches out (too young to understand long-term consequences). This meant turning around and driving right back to the ER for another four hours of fun (she got the last stitch out before we made it back to the ER). This put the return considerably after midnight; and any store that might carry pediatric arm immobilizers was long closed. When minutes count, two day shipping isn't much of an option. Spending more money wasn't attractive either, as we have just spent $250 on our ER visit co-pay.
These "Arm Immobilizers" were made from stuff I had laying around; I didn't buy anything for this project. They get the job done; D3 can't bend her elbows. This means that she can't get her fingers to her face. Like the commercial model, they are easily removed so that she can go to the toilet or feed herself while being supervised. Total cost for this build would be about $4 (mainly for the Velcro, most of which will be reused).