My son is nearly 16 now; his horseshoe kidney was discovered during an ultrasound when he was an infant. He had vesicouretal (sp?) reflux (this is common with this in the early years) and needed to be on an antibiotic until it corrected itself (good doctor not rushing in to surgery!!!). The antibiotic prevents potential kidney infection from the reflux.
We have been advised against contact sports for him due to increased risk of trauma because of the location of the fused kidneys in the pelvis. I did have him go to Taekwondo (with only noncontact sparring - the teacher understood) which taught him to effectively block his body with his arm if needed. He started this young and does this almost instinctively. I think this was worth the effort; there are probably other ways to teach this as well but this made it natural for him.

thank you so much for sharing your story. that is a great thing that you have done for your son and no doubt that i will be following the same path with my son and some kind of tae-kwon-do. my son just turned 3yrs old and that is my biggest fear of mine that he will get hurt. i dont want to baby him and not allow him to do everything his older brother and sister do either. i rarely even think about the fact thst he has only 1 kidney until i see his older brother jump on top of him and i go into serious panic mode while they are both laughing.
i have tried to explain to my other children to be careful but i dont know if they are just too young and dont understand or if they keep forgetting or just dont care.

I am the other horseshoe kidney. As the youngest of 3 my brother beat the hell out of me. I was also an avid horseback rider until a seriously fall ended all that. I was diagnosed when I was 35 and advised that I should not ride anymore. I actually was planning on going back to it too. The biggest problem I have is that unlike most horseshoe kidneys, my kidneys share a ureter and a blood supply - ergo if one gets infected I am in TROUBLE! Good luck with your sons.