I might be controversial but I wouldn’t use a body protector until the child is ready to be off lead rein. Body protectors, for all they will provide protection in a fall, they also inhibit development of balance and coordination as even a properly fitted one will restrict movement. It’s a risk/benefit decision for every individual parent but a child being led is at much lower risk of a high speed fall from significant height compared to one under control of their own pony.
I also agree my child wears one for jumping/ XC at PC as it's compulsory but nowhere else she has great balance and is also tiny so it really impacts movement. We have aerowear it cost £55 and is up to standard