Is it legal to kill or harm children in the purge? It says that ONLY LEVEL 10 government officials are immune... but I dont know if there are more rules about children stated in the movies and shows.
It says it right at the start of the first film; "Any and all crime".
I think we can reasonably assume that includes murdering a child, unless said child happens to be a Government official of rank 10.
Purely for the record, in the first film, the antagonist tries to kill the protagonist's children. No attempt is made to dissuade him by reading him the rules.