If I have a trust and want to add a request to it, do I have to go back to that attorney to put in that amendment or can I just add my request

I want to put down that any monetary remains from my estate be divided among my daughter and my grandchildren evenly (3 ways) without having to put that amount directly into my daughter's name. If the money goes directly into my daughter's name only, then my son-in-law would have his name on that portion as well. My concern is if my daughter gets the full amount, which then goes into both my daughter's and son-in-law's account, then my daughter dies, my son-in-law gets married again, then he dies, the new wife COULD keep all the money and would not necessarily leave any to my biological grandchildren. So can I just add a document into my trust stating that the money goes only to my daughter (if she is living at the time) and that she would divide it among her 2 children (1/3 to all) without having my son-in-law's involved with that money. (I have seen this happen to 2 people already where the biological grandchildren didn't get a penny.)