I quite often take photo's of other peoples animals. But they are taken from a road/footpath or something, never close enough to be called trespassing. I think the only time I could have been accused of trespassing was climbing into a dyke (empty stream thing) to take a photo without the fence in the way. Round here the dykes are generally considered part of the road etc but I am sure it could be disputed.
And come on, don't you all get that little rush of happiness when a little girl sees your horse and bursts into a massive grin! If she has a camera and took a photo would you be upset?