Working on mechanics is fine but only if the younger kids realize they are in the batters box for one thing and that is to hit.
Sounds obvious but coaches often shoot themselves in the foot in stressing "get a good pitch"; "lets swing at strikes", "stay back, wait on it" etc that kids get caught up in everything but actually hitting. They are trying to be too perfect because often coaches pound that into their heads. The kids easily get into the "wait to see if it is a good pitch" versus thinking "it is a good pitch until proven wrong". The coaches reaction to a kid swinging at a "percieved" bad pitch also influences a kid; baseball coaches are bad about 2nd guessing from the dugout about stuff they are often clueless about. Ask the kids what is their favorite pitch and location to swing at...I'll bet money you'll be suprised by at least half the answers at that age; most of them really don't know exactly what their favorite pitch location is.
Back in the Ice Age, during BP we were taught to swing at everything we could get a bat on so that we developed some bat control and the ability to adjust. We also learned real quickly what pitches we could not reach or hit well. BP wasn't at game speed so it wasn't hard to do but come game time, nearly all of us had the ability to always get decent wood on the ball and expand our effective hitting zone. Playing Pepper was something else we did religiously that aided bat control.
Job #1...get them to understand they are to swing the bat when in the batters box. They are hitters not umpires. Whats the worst that can happen...strike 3 looking or swinging? Might as well swing. Your reaction to a swinging 3rd strike is important so practice what you preach.
Gotta run