If you're just doing training...
for soccer
-basic passing 2 touches and then 1 touch.
-headers back and forth to get them used to it.
-Free kicking was fun. Although you might want to make them just do PK's if they're u12
For soccer use small games. 3 v 3 with 4 - 5 minute periods are good. Have asst. coach if you have one and parents keep score and add those points to each player on the scoring teams. i.e. if one team scores 2 and the other 1 than all the players on the team with 2 scores get 2 points. all the players on the other team get 1.
Rotate players around so everyone plays with everyone. A good way to do this is have players count off, then 1, 2 & 3 v 4, 5 & 6 etc. then say 17 (or your highest number), 1, 2 v 3, 4, 5.
If you can get parents to keep score get several small games going and use a whistle to end the game (don't use it for fouls, that gets confusing). I usually don't call fouls in small sided games just coach the player that fouled after the time is up on why they would of got called.
The important thing is you don't want to be the one keeping score. You need to watch the players and take notes. When you are done the kid with the most points are going to be your most aggressive players. Some will be your strikers. Watch though the kids with the least may be your best defenders. And watch for foot skills, passing skills and defending skills.
U12 is a great age. They are at the point where they can start understanding the tactical parts and position responsibilities of the game. I coached in a small league so we had ages 9 - 11 in U12. I didn't expect the 9 year olds to grasp as well but still required them to.
Are you coaching rec or select? I coached rec which had a 50-50 rule. All players were supposed to play 50% of every game. I am responsible for changing that rule to all players will have the opportunity to play 50% of a game. If they didn't hustle in practice I didn't start them. If they didn't hustle in a game I would bench them for 5 minutes. If they didn't hustle after that they were done for that game.
Pissed some parents off but when we started kicking every ones asses they decided it was a good idea, as well as the league.
If you are talking about little league football player, evaluate for speed. Find your 11 fastest kids and teach them to play. Fill in reserves accordingly. ALWAY make sure your top backs can play QB as well as RB.
For defense, put your fastest guys out there and blitz everyone but the CB's on every play. Little league passing sucks and the rare completion can be made up for in sacks, TFL's and int's.