Burnout said: They'd totally get slaughtered by the first yukkuri they meet.
Only if the author/artist wanted that to happen. And while I have no idea what the text is saying, seeing as there's no violence or drama going on, I don't think the author wants that to happen.
The little ones are Marisas-turned-Reimus because in return for letting pregnant Reimu stay at his house, Mister made Reimu promise to kill any Marisa-type babies she had.