Kids under the age of 12 who are playing football typically need basic defensive schemes that they can understand and follow, regardless of how the play goes. The best defensive scheme to use depends on the skills of the players, the opponent's tendency to pass or rush, and the areas where you have been weak in the past. Defensive schemes are typically named by the number of defensive linemen in the front and the number of linebackers behind them. The three basic schemes vary in how they allocate players to these positions.
Because youth players do not pass much, or very successfully, a 6-2 defensive scheme works well for young football players. In this setup, you have a defensive line of six players. Behind them are two linebackers, aligned behind two small gaps between pairs of defensive linemen. Two of your remaining three defensive players act as cornerbacks, and the last is a safety in the far back. If you prefer, you can have all three as safeties to make it easier to defend against long passes. From this defensive scheme, you can blitz with the safety up through the middle or the linebackers through the gaps. Its main weakness is that the middle of the field is vulnerable because you don't have a center linebacker.
Very similar to the 6-2 defense, the 5-3 puts in a center linebacker instead of one of the defensive linemen. This strengthens the team's ability to defend against passes, although the defensive line cannot block a rush as well or put as much pressure on the other team's quarterback. You will find that there are definitely holes in the defensive line, but the linebackers can come in to help if the other team is rushing the ball. Also, this defense can be very weak around the sides because the line is not as spread out and the cornerbacks or safeties are not usually involved much in rushing plays. However, if you know the other team is planning to pass this play, the 5-3 defense guards well against it.
This defensive scheme is named for the number of defensive linemen and the shape that the other four players form. This name makes it easy for the players to remember how to line up if you call them into these positions before a play. You have seven defensive linemen, which provide a lot of strength to block running plays and, if given the opportunity, put a lot of pressure on the quarterback who is planning to pass. Plus, the seven players can typically pair man-to-man against the offensive players who line up across from them. Behind that, you are a little bit weak. You have a middle linebacker who provides some protection for the short pass or if the running back gets through. Behind him and to his sides, you have two corner backs that can protect you against passes to the sides. In the back, the safety gives you protection against a long pass.