Long term, it depends upon what the constraints are upon population size.

For example, if it happens in an isolated village where the food supply varies from year to year due to drought, and the next year the food supply will be so short that some children will starve to death, then the premature death of one child the year before the famine will have no effect upon the number of villagers alive 20 years later.

The same dynamic applies, if a large factor in deciding whether to have a third child is whether the parents can afford to educate that child, and the cost of education depends upon the number of children competing for a limited number of school places.