A very common misconception is that soccer is a 'non-contact' sport. Nothing could be farther from the truth. When you have two children running full speed after the same thing (the ball), contact is going to happen. When two players are running step for step after the ball, some arm/hand jostling is to be expected. If a player lifts their elbow out or shoves the other player, it will probably be called. If a player blocks the ball and the other player trips over it - it's legal (a clean tackle). If they hit the opponent's legs first, a trip will be called. Generally, if a player touches the ball first and in the process the other player trips over their legs - it's usually legal. Keepers tend to have a wide latitude in terms of going after the ball. If they dive on a ball and the attacking player trips over the and goes flying - it's legal.
So while it's an American pastime to hassle sports officials - please don't. Soccer referees are heckled constantly, often by parents who are mistaken about a given rule. Referees miss calls just like any sports official, but more often than not, they get it right. So if you're SURE a call was missed, talk to your coach about it - they may have some insight to share that will help you better understand why a given call wasn't made.