That's an interesting point. However, one could cry "RetCon" on the part of the VOY episode w/regards to what Seven said.
Regardless, you could easily put a newly born artificially incepted Borg child in a maturation chamber. I'm sure they were looking for more of the horror angle, tho. Imagine what it must be like to be a newborn, toddler or even preschool child that is assimilated by the Borg. To me, that's even worse than being an adult and being assimiltated!