Can you prove there were children on the earth when God brought the flood? No, of course not. We can only assume either way.
Noah preached for 120 years while building the ark. God knew, of course, that the people were not going to turn. It's more than possible that He caused all those who were going to die in the flood not to be able to have children. Any children there were at the time Noah started building were more than grown.
So you're going to read something into the Bible that isn't there. It appears though that life right up until the rain started was pretty much normal for that time:
For as in the days that were before the flood they were eating and drinking, marrying and giving in marriage, until the day that Noe entered into the ark [Mat 24:38]One would think that "not having children" would be noticeable enough to have rated at least a mention.