Make sure you have a contract in place for your portrait sessions too. You will need a signed release to post/share/use any photos you take of any portrait clients.
And yeah, liability insurance and equipment insurance is a MUST HAVE for equine photographers. Think about your most expensive vet bill and multiply it times 100 or 1000. You could be liable if a horse is spooked or injured for any reason at one of your photo shoots. Please understand that even if your friend and their parents don't sue you, their insurance company sure will!! We tend to forget how dangerous horses can be and how easily they can hurt themselves or the people handling them -- or innocent bystanders. I hope nothing ever happens while you are photographing a horse, but you want to be covered in case it does.
Portraits are more what I'm aiming at. But some of these comments make photographing sound like such a terrible job!