“ce sont” should be used but when we talk it’s OK to say “C’est”.

more informations if you can read french

“c’est”, like “on”, tends to be a neutral even in the number (in popular langage). E.g. Only very literate people would write “vivent les vacances” instead of “vive les vacances”, despite only the first being correct. Similarily (but less extreme) at oral people easily say “c’est + plural” instead of “ce sont”. You will be considered as slightly picky if you use the correct grammar, especially at oral (possibly even for “c’est des choses qui arrivent”). The use of “c’est” often is a lot like “voici”, finally.

C’est mon fils et ma fille is not just familiar but an accepted and common style.

There are rare cases where using Ce sont is mandatory like where it expands a previously expressed plural:

Je parle espagnol et anglais, ce sont des langues que j’ai étudiées à l’école.

and not

… c’est des langues…

Otherwise, despite being less formal French, it is not grammatically incorrect to use c’est instead of ce sont. There are even cases where the singular is mandatory:

C’est dix Euros (The price is €10)

Ce sont dix Euros

C’est nous qui… (It is all of us who…)

Ce sont nous qui…

Source: Académie française : C’est / ce sont

Here is also an interesting article about this illogical use of the plural with a singular pronoun :

Ce sont des anglais – Un accord avec l’attribut ?

• part 1

• part 2