But I'm not shure if giving +100 attack would be the right way to depict it.
If horses turn away before charge, then most of them survive and sometimes cav charges without order or the AI does BS by doing a frontal charge into pikes. In that case it would be less realistic that you lose all you cav, so may be it would be even more realistic to decrease the attack and charge of the cav drastically and increase the def of cav and pikes. Like that doing a frontal charge would be kind of senseless and the cav has to turn away to try their luck again. But I doubt that even THAT is possible in the engine.