「お母さんが頭を下げて試供品を受け取る + の + と、まる子がぴたりとさわぐのをやめる + の + は同時だった。」
The basic structure of that sentence is:
「AとBは[同時]{どうじ}だった」 = "A and B took place at the same time."
A and B, in this case, both happen to be mini-sentences, don't they?
A: 「お[母]{かあ}さんが[頭]{あたま}を[下]{さ}げて[試供品]{しきょうひん}を[受]{う}け[取]{と}る」 = "Mother bows as she takes the free sample"
B: 「まる[子]{こ}がぴたりとさわぐのをやめる」 = "Maruko suddenly stops making a racket"
Now, in order to say "A and B happened at the same time." rather than "A happened at the same time B happened.", would one not feel like nominalizing both mini-sentenes? A Japanese-speaker would, so he would attach his go-to nominalizer 「の」.
The sentence itself is in the past tense - 「同時だった」, but one would need to notice that the two actions are not - 「受け取る」 and 「やめる」.
Grammatically speaking, 「同時だった」 at the very end is the main verb phrase of the sentence. The far longer part that precedes is only the subject of the sentence.