I'm going to explain this whole kid situation the best I know how which is using sports metaphors and basketball lingo. Even though I'm a hoops guy, I think anyone that has ever played or enjoys watching sports can understand this. These are things other people fail to tell you before you have kids so I'm trying to make it easier for parents out there as they consider having 1, or even 1 more.
Open Jumpers and Fast Breaks - 0 Children aka 0 vs. 2
This is married life. You might miss a few shots and the occasional wide open layup but in all reality, this is not real life for any couple eventually planning to have kids. Enjoy your DINK (Dual Income No Kids) lifestyle and get that fun time in now. Go on as many vacations as possible, go out to dinner, get drunk at BBQ's, have lots of sex, etc. Do what you gotta do because this time is limited. Go out for 81 points on 46 shots like Kobe in 2006 because you are going to get old quick and rack up some major injuries in the near future. Enjoy your prime while it lasts.
Full Court Press and Corner Trapping - 1 Child aka 1 vs. 2
Now you have your first child. Good for you. Your life has changed but in all honesty, things are still pretty damn easy. Anyone that says having one kid is hard is delusional. You will look back on this time and realize how good you had it. You might even whisper to your oldest when you are alone with them and reminisce about the good times and say "Remember when it was just me and you? Wasn't that way more fun?"
When both parents are forced to play a little D, you can prevent this one child from doing anything bad or life threatening. You are exerting little defensive effort like you are the parental version of James Harden. Even when things get hectic, you always have help D. You can trap them in the corner when they start crawling or walking and they will never get to break anything or mess up your house. This child will most likely be the most well behaved and smartest because they will get one parent's full attention, if not both. Things are good.
Worst case scenario, one parent can cherry pick on the offensive end and let the other go Man to Man. Cherry picking allows you to get some quality alone time if needed like going to the gym, binge watching Netflix, playing video games, getting errands done, etc. You are still racking up points, but you have more contested shots so your shooting percentage might slightly decrease. You are still scoring 20-25 per game, although you are at the tail end of your prime and your stats will go down over time.
Man to Man Defense - 2 Children aka 2 vs. 2
$h!t is getting real now. No one has free time and you have no subs. You still have to run a hard man to man, often starting full or 3/4 court but you are running on fumes. Your back and body hurts from foot fires (quick feet to avoid toys and other baby related landmines) and defensive stances (aka bending over picking up toys, carrying crying children, breaking up fights, etc.). This is the NBA Playoffs every day. You can't take a day off because your main goal in life is to make sure you keep these children alive and they don't break a bone or lose an eyeball.
If one parent has to leave for any extended amount of time, you have to drop back into a 1-2 zone. You might be able to break up a few passes, and block a few shots, but most likely, you are getting scored on often and are committing crazy fouls just to stop the game and catch your breath. Don't be against trash talk and hoping to provoke one of your children into a temper tantrum so you can give them a Technical Foul and send them to their room or the time out chair. It's not pretty, but at least you can catch your breath and go back to a relaxed man to man.
Zone Defense - 3 Children aka 3 vs. 2
You are officially outnumbered. All you can do is hope to prevent a score on every possession but most likely, you are getting blown out almost every game. You are starting to look like the 2014-2015 Sixers except you are not trying to tank. You just lose every game even with your best effort.
Your head needs to be on a swivel constantly as you run the baseline or cover the wings or top of the key. No matter how you set up the zone, you are always leaving a man open and you are screwed if you lose track of the scorer or hot shooter. If they make the right pass, you are pretty much giving up a bucket 70% of the time.
The scorer is usually the youngest child because they are way too good at dropping behind the zone or to the far corner and out of your peripheral vision. They have quickly learned how to beat the zone in their short career by using just a few quick pump fakes and go to moves. They have never experienced a true man to man or trap for more than a few moments at a time, so getting open is like Child's Play (yes, I said it, and this cliche was finally used properly).
They are often the quickest and dirtiest player, as well. They are always out to make it hurt as much as possible. The only thing you can do is know where the ball is at all times. If you aren't quick enough on your feet later in the game/day, at least keep an eye on this 3rd kid and try and see man and ball (aka dangerous targets like stairs, sharp objects, TV's, bathrooms/toilets, etc.).
You are essentially in a Prevent Defense, to bring in some football terminology to the mix. You know that you are giving up major yardage each down but you are just trying to prevent a late score before bedtime. You aren't winning the game. You are just trying to keep it from being a blowout. I suggest calling for subs or backup as much as possible but most players won't want to come in to the game. They see your pain and don't want any part of this blowout. All of that help that you got on Defense when you had just one kid and didn't really need it, is few and far between. You are pretty much on your own and all family and friends think you are crazy for deciding to even participate in this game.
Crazy Defense - 4+ Kids aka 4+ vs. 2
There isn't even a defensive analogy for this situation. If you have more than three kids, you are just crazy. Waive the white towel now because you are going to get buried by the blitz on the first play of the game. Fast break dunks on your head all day. You will feel like this guy after D. Rose just hit a ridiculous game winner in OT.
I think that about wraps it up but if you can think of any other scenarios or relevant defenses that I missed, let me know in the comments.