I have three daughters and we have tried various makes. Rodney Powell is the best in our view and can expand a little as child grows. Getting it properly fitted is the most important part, long enough to cover the back but short enough to allow rider to sit properly. Like riding hats, an ill-fitting one can get caught up and cause more damage if you fall. Hope this helps.
They certainly go small enough - DD had one when she was 2. I'll have to research the brand because we gave it away years ago. Now she's in a Rodney Powell which has easily lasted 3 years because they adjust with velcro at the back (she loves the zip up front). Highly recommend them...
Assuming you are talking about your son falling off a pony, you need to go to your local tack shop (making sure that the sales person is qualified to fit such things) and get one fitted. DD has an Aerowear one that I got her last year and will do for this year too. Think it was around £80-£90. I don't know how small they go but DD certainly had one at around 4 when she was coming off the lead rein a bit.
My almost 3yo had a fall this afternoon and it's made me panic a bit. I've googled body protectors but there seem to be so many out there I thought I'd get some advice first from people who might use them.
Do they actually exist for a 3yo? How quickly do children grow out of them? Is there a particular make to seek out/avoid?