Well, its a fact that the way we explore science is much more intense than before. There are many instances when old theory has to revive due to the new understanding. For example, no body wanted to accept the origin of quantum mechanics to replace the classical newtonian mechanis, but also no body could stop it. But this instatce of Pluto is really deep into our thinking, it may be hard to explain a kid that, what he learnt till now was wrong and there are only 8 plantes. I would explain by saying that, before people misrcognized a big rock as a 9th planet, but there are thousands of such rocks, so it cannot be in the same category as other 8 plantes. A less name to memorize......