I haven't yet coached at this specific age, but I coached Babe Ruth and AAU baseball for 7 years (so mostly 14-15 year olds) and am now coaching my own two boys up through LL (they will be in A and AA this Spring), as well as umpiring in my local league for the past 4 years, so I call quite a few Majors games each year and as such have a decent feel for what I think they need to work on!
Obviously at this age you need both repetition of the "basic" drills & skills (ie taking grounders), but you can start mixing in more advanced techniques as part of those to mix it up and keep it fresh. I think some emphasis on infielder footwork is appropriate at the age 11-12 level, as far as expecting them to work on their "route taking" on ground balls with the goal of getting square to the ball when fielding (or determining they need to just take it on the run/backhanded/whatever). I think you could also do a specific ground ball drill for the infield that involves how to confidently charge a slow ball and make the throw. It may be helpful in such a drill to utilize a couple runners with helmets to demonstrate to the infielders how much time the do or do not have to make such a throw. Part of the point of this drill is to also know when to just "eat the ball" (aka not throw it away needlessly down the RF line) and you want to acknowledge the kids for making good decisions on such plays in practice - as we all know those plays can sap moral/give the other team free bases/runs. A specific drill you can do for the 3rd baseman & SS is for when you have a runner on 2nd and a slow roller to 3rd, the SS should move in behind to cover 3rd so that if the 3rd baseman sees no play at first he knows to immediately look back at 3rd for the runner over running 3rd - use a baserunner in this drill and tell them to sometimes take a big turn (to draw the throw) and other times to stay on/close to the base - again, part of what you are teaching here is proper decision making on throw/no throw judgements.
I think drills that also work on making the infielders (and/or outfielders) practice the skill of estimating the ball's path on a pop up or fly ball and putting their head down and running to that estimated position and then picking it back up again, getting behind the ball (again shoulders square to the play) and set in good two handed catching position is key - with the outfielders this drill can also then add getting the ball out of glove quickly and stepping into their throw.
With your kids who play the 2nd base/SS position I think you can also do some drills on receiving the catcher's throw at 2nd base / receiving a throw from an outfielder for a play at 2nd / receiving throws from each other for plays at 2nd. You will need a coach doing the throwing of balls to 2nd base (mix it up on purpose to work on different techniques - some right on the bag, others wide/high/bounced etc), and another coach at 2nd ideally to coach the proper techniques/give pointers after each play. The things you want to emphasize in these drills is reading the flight of the throw, the concept of proper body position to make the tag (knees bent, shoulder's square, glove up, position slightly behind the bag), knowing when to abandon chance of a play and come off 2nd base to just stop the ball and how to execute a proper "slap down tag" on the leading edge of the base where the runner's sliding foot will be coming in. You can also explain the concept in these drills (as to why they should be positioned above/behind the bag to get the throw) about not blocking the runner from the base without the ball (obstruction).
Hope that helps - good luck Skip!
