You wrote:

I was trying to explain a “compound sentence” to my son, which referrs to two clauses.

Here’s your definition of a clause:

Clause
A clause is a group of words containing a subject and verb which forms part of a sentence. The first sentence on this page is made up of two clauses: the first clause from “A clause” to “verb,” the second from “which” to the end.

Perhaps the “which” is serving as a noun to form the second clause in this case? Otherwise, isn’t “clause” the subject of both of the parts, and thus the sentence is actually one clause and one phrase???
Thanks!
J & A
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“It is a sobering thought that when Mozart was my age, he had been dead for two years.” -Thomas Andrew Lehrer

(I love Tom Lehrer!)

Actually “which” is the subject of the second clause. The pronoun “which” replaces “clause” while also doing the job of a relative pronoun. The second clause is part of the participial phrase beginning with “containing,” but the sentence does have two distinct clauses.