Halakhah sur Parah 3:13

And if you ask, "How we could ever find a man who had never been rendered impure by a dead body ever" - the Sages said (Mishnah Parah 3:2) that there were courtyards in Jerusalem built on rock [and] below them was hollow because of the [concern over the] graves of the deep. And they would bring pregnant women and they would give birth there and raise their children there. And when they wanted to sprinkle on the priest that would burn [the heifer], they would bring oxen - as their bellies were inflated - and they would place slats (literally, doors) on their backs and the children would be made to sit on top of the slats, so that there would be a 'tent,' separating between them and the ground, because of the [concern over the] graves of the deep; and there would be stone cups in the hands of the children. And they would be brought to the waters of Shiloach, and they would descend there and fill [the cups], as there was no concern there over the graves of the deep, since it is not the way of people to bury in rivers. And they would return and mount atop the slats on top of the oxen and go to the Temple Mount. And they would descend there and walk upon their feet, since all of the Temple Mount and the yards' underneaths were hollow, due to the concern over the graves of deep. And they would walk to the opening of the [Temple] yard and take from the ashes and place them in cups; and they would sprinkle the priest that burns [the heifer]. And the children would immerse due to the concern that they had become impure with another impurity. And all of these things are from the extra stringencies of the heifer. And it was burnt on the Mountain of Anointing (Mishnah Parah 3:6).

And if you ask, "How we could ever find a man who had never been rendered impure by a dead body ever" - the Sages said (Mishnah Parah 3:2) that there were courtyards in Jerusalem built on rock [and] below them was hollow because of the [concern over the] graves of the deep. And they would bring pregnant women and they would give birth there and raise their children there. And when they wanted to sprinkle on the priest that would burn [the heifer], they would bring oxen - as their bellies were inflated - and they would place slats (literally, doors) on their backs and the children would be made to sit on top of the slats, so that there would be a 'tent,' separating between them and the ground, because of the [concern over the] graves of the deep; and there would be stone cups in the hands of the children. And they would be brought to the waters of Shiloach, and they would descend there and fill [the cups], as there was no concern there over the graves of the deep, since it is not the way of people to bury in rivers. And they would return and mount atop the slats on top of the oxen and go to the Temple Mount. And they would descend there and walk upon their feet, since all of the Temple Mount and the yards' underneaths were hollow, due to the concern over the graves of deep. And they would walk to the opening of the [Temple] yard and take from the ashes and place them in cups; and they would sprinkle the priest that burns [the heifer]. And the children would immerse due to the concern that they had become impure with another impurity. And all of these things are from the extra stringencies of the heifer. And it was burnt on the Mountain of Anointing (Mishnah Parah 3:6).