I've said it in previous threads on the topic of summer ball, but have your sons wait as long as possible to sign their contract for summer ball. Basically what @adbono said.
So much can change between the fall and May. For example let's say you're a kid doesn't have a good year and is told to find a new home. You don't want to play in a league that coaches/recruiters don't recruit from. For example the Hampton's league. Nice place, but difficult to get to. Coaches probably can't see many players that it makes it worth it to travel there.
Last year my soph son didn't play in the fall because he was rehabbing from an injury, he didn't get assigned (and didn't want to be) assigned anywhere. Had a good start of season, transitioned to Friday starter, and was either going to play in the NECBL or CCBL (I'm sure would have been a temp contract then to the the NECBL). Got hurt, had TJ.
This season he's hoping to be back by the middle of conference play, and is assigned to a NECBL team for the summer. Who knows, he may not face batters until the summer.
Also seems like summer ball is different now than it was 3,5,7,10, etc years ago. With the portal the way it is, coaches don't want their players poached over the summer, and kids don't want to play 50+ games after playing a full college season. Your son has to do what's right for them, while not pissing off their college coach....