First, the underlying premise of the argument is significantly flawed. The secret service has provided protection for the children of every president since William McKinley as they could obviously be a target of those hoping to undermine presidential power and national security.
Second, the ad shifts focus halfway through bringing in the issue of tax fairness as some example of hypocrisy. This only muddies the waters.
Third, and most importantly, this speaks only to their core audience of gun owners who are worried about an erosion of their second amendment rights. In this debate, the people who need to be convinced are non-gun owners and others who don't see a need for civilian versions of military weapons and magazines that hold 30 rounds or more to be in the hands of anyone other than police and military.
In targeting the president and his children, the NRA has given gun control advocates a gift which they will use to paint the NRA as extreme and out of touch with mainstream America. It's lousy strategy wrapped in poor execution.
If they keep it up, they'll lose this fight.