No boobs, no boots. Of course, that didn't work for those of us who remained flat-chested until after our junior years were over!
It's technically appropriate for a child showing a horse to wear jods, and if the child is also showing a pony, or is still in the middle of a growth spurt, jods are definitely fine. Once the child has some height, or is moving into the Children's or Junior hunter division, tall boots complete the picture a bit better. Oh, and once the child is in tall boots, no pigtails or bows.
The function of garters is to keep the jods from riding up around the knee and causing saddle sores, especially if the elastic boots clips break or let go. They also help create a better silhouette for the leg. Otherwise you can get this little "wing" of fabric behind the knee.
Life would be infinitely better if pinatas suddenly appeared throughout the day.
There are as many answers to that question as there are people to answer it. My own DD moved into tall boots this year, and she's 10. She does show a horse, not a pony. Another rider moved up at 9, but she's taller than my DD who is over 5' already. Both looked silly in jods as you can't tell they are younger when riding. It was about the look, and that they ride better in boots than jods. We have another girl at our barn who is 10, but much smaller than these girls and still in jods.