Unless they had children while in the garden but I do not believe that is true.
Genesis 2:1
And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.
Eve could of been pregnant while in the garden but I do not believe that is true.
Which I believe when Adam and Eve were ousted from the garden that is when Adam knew Eve which means they had sex.
It could be that they were not in the garden long before they sinned not having time to have sex.
It could of been the day after God commanded Adam not to eat of the tree that they sinned.
If they were in the garden for a while before sinning why didn't they get going on the repopulate the earth.
It could be they were not supposed to have children while in the garden with Eve being a help mate for Adam but not having children yet.
For God said replenish the Earth but if they had not sinned they probably would of been assigned to the garden and not ventured out.
For the garden was made for them to live in that habitat not in the habitat of the world.
So God knew they would sin and be ousted from the garden where they would then fulfill the mission repopulate the earth.
Which God did know they would sin for Jesus is the beginning of creation, and without the plan of God to come in the future in flesh He would of not created anything He created, and the Lamb was slain from the foundation of the world.
So God already had the plan of salvation for mankind before He created Adam and Eve so He knew they would fall.
It could be before the fall being in the garden having children was not part of the plan and not going to happen in that environment.
That is what makes sense to me that while they were in the garden in that setting they would of been confined to the garden and would of not ventured outside the garden.
So having children in the garden was not a part of their lifestyle yet until they were ousted from the garden and put in the world then they could have children in that setting and repopulate the Earth.
The garden was only for Adam and Eve not their offspring and they did not go out in the world.
When they sinned and went out in the world then they could have children fulfilling the command repopulate the world.
Having children in the garden did not apply for they were not in the world.
God said replenish the earth not replenish the garden.
The garden was a stand alone place only meant for Adam and Eve and children was not part of the plan there.