I looked up the video again and apparently there are two videos.. one in which X is sitting on his Dad's shoulder and he points at someone in the crowd and appears to say "shut your mouth".. and the other has X standing next to Trumps desk and says "sush your mouth"

I'm sure he's mimicking something he's probably been told many times. It's funny, but again, leave the kid at home or with a nanny while you're doing official business especially when televised.