Sorry everyone, but there isn’t even a clear source for a child not to say kaddish during his parents life time.
Many kehillos, such as belz, and many sefardishe, are not makpid bichlal.
Someone asked the Steipler zt@l, and he said that since today it is not usually done, and people migth assume that he has no longer got arents, it is not a good idea.