Well, I think some of the kids in the picture that was posted probably think the NRA has lobbied to prevent gun control legislation from getting passed, and see that as partially responsible. And I can only imagine that some think the President of the United State has some say on what happens with regards to policy in the nation. But I’m only speculating, cause I’m not in the picture that was posted, and I don’t blame Trump or even the NRA.
If the kids were properly informed, they'd understand both Trump and the NRA don't oppose increased restrictions on the ownership or acquisition of the type of weapon that was used in that massacre. The NRA in fact has never stood against gun registration (they often sponsor gun registration drives) and as far as I know Trump isn't opposed to increasing age restrictions, enhancing background checks, limiting certain kinds of gun clips (like bump stocks) or arming personnel in schools to prevent more of these types of heinous tragedies. The kids are passionate, but uninformed.