As far as I understood, "Ce sont" should be used, when the subject is plural (there are exceptions regarding quantities etc.), "C'est" otherwise. I know that in familiar speach it may be acceptable to always use "C'est".
The question arises, what's considered a subject that qualifies as plural gramatically, when facing enumerations. It seems that an enumeration of singulars is normally treated as a singular subject, but that doesn't seem to be universally accepted.
I.e. "C'est mon fils et ma fille" and "Ce sont mon fils et ma fille" can both be found at google, which doesn't help. That may be because of the exception regarding "C'est" in familiar speach, but it seems to be the more dominant version, too.
Combinations of singular + plural seem to be generally treated as plural, independant of the order. The "C'est" exception in familiar speach ruins any certainty there for me, though.
I could imagine combinations that are considered a singular entity or with a predominant singular (for example the king and his subjects) are officially treated as singular?
Pointing me to an authoritative reference would certainly be helpful.