Index, let's look at another scenario.
If Couple B have those 2 kids in their 30's, and their kids have their kids in their 30's, that spreads things out. If couple B sells their house and moves into a house/condo in a dense urban area they also reduce their effect.
Think of Couple D, who marry young and have lots of kids, it is their culture/religion/family tradition/whatever.
Let's start them in the year 1980, they are all born.
In 2000 Couple D get married at age 20, over the next 10 years they have 6 kids.
In 2010 Couples A and B get married at age 30.
We know Couple A has very little impact so I am dropping them.
In 2020 at age 40 Couple B have a 5 year old and a 2 year old and are done. They have a moderate house, and one, maybe 2 cars.
In 2020 at age 40 Couple D have 5.5 kids, 9, 7, 5, 3, 1 and one on the way, and a really big house and a big minivan or equivalent and at least one other car.
In 2030 Couple B are still in that house with a 15 and a 12 year old.
In 2030 Couple D are still in that big house and have 6 kids, 19, 17, 15, 13, 11, and 9 and have that big van and at least one parent car plus 2 beater cars for the 2 older teens. Plus all the extra resource use for 8 people versus 4.
In 2040 Couple B have moved to a city apartment since the kids are now 25 and 22, first is done university and is working, second is about to graduate and start working. Since their careers are just starting the two B kids are going to be living in apartments with roommates while they get their careers started. City living means Ubers and car shares. So Couple B and offspring are 4 people and 3 apartments and no cars.
In 2040 Couple D's kids are 29, 27, 25, 23, 21 and 19. Since in their family you get married at 20, that means 5 of the kids are married. Kid 1 at 29 has 5 kids already, kid 2 at 27 has 4 kids, kid 3 has 3 kids, kid 2 has 2 kids, and kid 5 has 1 kid. Kid 6 is planning their wedding. So Couple D now have 15 grandchildren, plus those 5 married kids all have houses and most have 2 cars because of all the grandchildren
Overall, Couple B are not a huge problem, because having 2 children is still below replacement rate because of couples like Couple A. With a long generation time, Couple B's approach also means that population decline is gradual with no weird social implications.
Couple D are a huge problem - the large number of children and the short generation time are massively resource intensive. And odds are a lot of couples like them won't stop at 6.
And I know that people used to have lots and lots of children. They also lost lots and lots of children to disease and starvation.