[[Appendix:Proto-Germanic/kelþan]]
Fragment of a discussion from User talk:Rua
In A Handbook of Germanic Erymology by Vladimir Orel published 2003, the entry *kelþaz is listed as a neuter z-stem. As far as I know, z-stems were not productive in Old English. A noun from an unproductive class (like the z-stems) would be more likely to be moved to a more productive class (like the regular a-stems) rather than the other way around. It would not make sense if (ċild, ċildru) were originally an a-stem then switched over to an unproductive z-stem.