The very first link in a Google search - several states have explicity sentences for the murder of children, either with higher maximums, higher minimums or transforming things which would otherwise be Manslaughter into the same as First Degree Murder. (Just search for “Child” in that page to find those).
Also check this paper. Even though it’s about gender rather than age, you can find the point I made earlier about “vulnerability” for example at page 435 section B.1 as well as explicity references to children in the various Sentencing Aggravatory Scales under Apending I (from page 464) explicitly under scale I and IV and implicitly in other scales (i.e. Scale 3 - Heinousness) which whilst they don’t prove that child killing explicitly is deemed more heinous than others, does prove my point that society has Heinousness criteria for Murder, disproving your “Murder is Murder” take.