My liability (for lessons & training horses) is an add-on to my farm policy. Fair warning, it isn't cheap! I know what I pay and given the avg price of a pony party, I'd have to do at least one a week just to cover the cost of the liability policy.
Look into ag/commercial equine insurance providers, they will be able to get you the best coverage and rates.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.