I have 2 sons, one is playing in college and the other has committed... both to D1 programs. The older one played primarily legion and very little showcase or "high exposure" events prior to committing. He did play in a State High School All-Star event that drew a lot of interest and one PG showcase event the summer before committing. He played other showcase events after committing just to see if we could generate interest for the draft. The younger one has played only legion.
I think you really need to assess whether the events they are in are getting them the right exposure and do you have the right support from your high school or summer coach that can help generate interest. If no colleges or scouts are showing up to their events, they need to go where they can be seen. In our area, we were getting pretty good exposure playing legion, high school, and a couple of local showcase events, and that is what ultimately led to playing in college. I'm sure that is not the case in all areas.
Another point is that if you are a big talent they will find you, regardless of where you play. One kid in our area played some showcase, but not for a top team and mostly locally. His senior year of high school, he had 20-30 scounts watching every time he pitched. One of those times, my son was the opposing pitcher and there had to be 30+ scouts watching.
So bottom line, it really just depends on your area, the degree of support from your coaches, and you son's talent level. You should also talk to current or prospective coaches and the parents of older kids in your area that have recently gone through this to get their pespective.