I know how you feel...except for me, the 19 months and up are just fine with the puzzles, towers etc...it's the 12-13 month olds who eat puzzle pieces etc. Currently I have three 12-13 month olds who nap in the mornings, so I do puzzle/art time while they sleep. As for the tower building etc...I just keep telling them no and removing them to another toy or showing them how to help. At 19 months they are quite capable of understanding that they should not detroy other kids' towers or puzzles etc. I would enforce the rules clearly and give time-outs for misbehaviour. Also, give them their own puzzles to do (wooden peg puzzles etc) and let them help build a tower instead of knocking it down. 18 months and up are more than able to join in these activities.