There are a fair amount of different defensive schemes or strategies a football team can employ depending on a myriad of factors. These might include overall philosophy, athletic ability of players, how much depth is available, down and distance, time of the game...
There are 3 basic defenses:
Man-to-Man: each player is assigned to defend and follow the movements of a single player on offense
Zone: instead of each player guarding a corresponding player on the other team, each defensive player is given an area, or a "zone", to cover
Prevent: A prevent defense backs up the coverage so far that it will often give up long yardage, but it makes scoring a touchdown in a single play very difficult
As our family has grown, it's funny how the defensive strategy so closely aligns with these three general football strategies.
Child 1 - this doesn't really count, is more like a practice game, and no particular defensive scheme is required, simply understand that whomever is closes to said child defends at that particular time. This is sort of like having a pet, just make sure you feed/water/change it, surely between two grown adults this can be handled without much fuss.
Child 2 - ok, now we're getting down to a game plan, for sure you can play man-to-man on this one. Honey, I'll take number 1 tonight and you take number 2. If things get too messy we can switch on the screen and pick up the other one, just don't forget to switch on the screen.
Child 3 - you are now out numbered and are forced to play a zone defense for sure, make sure you cover your area (chores, homework, carpool, etc...) and keep open lines of communication so as to not get confused with your area. And by all means don't get the hand signals mixed up or someone will be running free in an uncovered area of the field.
Child 4 or more - this is prevent defense the vast majority of the time, just drop back, keep everyone in front of you at all times, and don't lose eye contact, whatever you do, don't let that fast sneaky one get behind you! Again, communication and eye contact with each other is vital on this one as you are attempting to "herd" multiples in an area without hanging your teammate out to dry with unfair numbers.
Even within these schemes there are slight variations. Last night after coming home from work, it was evident I needed to run a cover 2 (ok, this isn't technically a cover 2 but it makes sense for this illustration) and drive Eli and Fisher to another part of the playing field as far away from mom as possible. While this did leave Melissa having to defend 3, they were fairly pre-occupied and not "in the play" too much while she was preparing our fixins for the evening. It gave me a chance to just go be silly like a 3 and 5 year old away from everyone. Sometimes culling a few from the herd is good for the spirit, laughter, and self preservation on my part when mom has that Ray Lewis crazy eyes thing going. I love a great defense, but a personal foul penalty usually only hurts your teammates and is detrimental the overall strategy.
What kind of defense are you running most days and under what circumstances does it change?