For example (Luraghi 2018): "More than simply indicating referential sex, Luraghi (2009b: 127 and 2011) makes the argument, with which Melchert (2014: 264) agrees, that an involvement in procreation may have played a role in gender assignment in PIE. It may have been a key factor motivating the split between masculine and feminine. Nouns that referred to male or female humans/animals were, for the most part, assigned to masculine and feminine, respectively (Lundquist and Yates 2018: 2095). For example, there was the masculine noun ph2tḗr ‘father’ beside feminine méh2tēr ‘mother’ and wĺ̥kwos ‘(he-)wolf’ beside wl̥kwíhxs ‘she-wolf’ (Lundquist and Yates 2018: 2095). However, words that indicated children and young animals were often neuter in the IE languages (Melchert 2014: 264), e.g. Gk. téknon, OHG kind, and OCS dětę (Lundquist and Yates 2018: 2095). The humans/animals to which those nouns refer are too young to procreate, so, despite being physically animate, they are grammatically neuter.
A questionnaire experiment determined that feminine anaphors were used more if Mädchen referred to, for example, an eighteen-year-old girl “as opposed to a two- or [twelve]-year-old one, where neuter pronouns were more frequent. These results show that speakers perceive biological sex as more important for adults than for children” (Corbett and Fedden 2016: 522). This supports Luraghi’s theory that involvement in procreation was a factor—alongside individuation—in gender assignment in PIE"
Words that refer to children and which aren't of neuter gender today may simply be of later origin, when the system stopped being productive. For example, the Russian word for "child" is rebenok, which today has masculine gender, but stems from earlier "rebia" which was neuter (a synonym of another word, "ditia", also neuter).