Paid assassins? Mexican drug lords? Please explain.
Mexico just a drive away. Businesses that aren't well know in their own small knit community which give them a reason to cross the border transporting goods back and forth. Businesses allow them access to chemicals that can be used for bad things. They didn't find any meth labs on the farm because this is a sophisticated operation and nobody would be that stupid.
Wanting to retire in Mexico suggests to me familiarity and connections to the area. The grandparents may have been approached by individuals in Mexico that do not take no for an answer.
Something that might reveal things is finding out if they bought a place in Mexico? From who?
The child could be in Mexico right now