It might be that the children grew like in the days of Hazrat-e-Adam (AS). Hypothetically (not)
At that time, circumstances were different. People lived for hundreds of years, and they had to populate the earth with human beings. Hence Bibi Hawa having a child in the morning and a child in the evening. But we're talking (i think) about a scenario in which the population has settled down, and there isn't a major concern of population