Muslims have no problem whatsoever in accepting that Jesus (pbuh) spoke as an infant in order to protect the honour of his mother.
Why should Christians find this action of the infant Jesus (pbuh) to be so objectionable to their beliefs?
According to the alternative and contradictory version of this story in the third chapter of the Koran, Jesus could speak in the cradle as a sign of His prophetic mission, not necessarily for the defense of his mother’s honour and chaste. In Surah 3 we can never see/hear Jesus speak for this purpose since that surah forgets to narrate that people accused Mary of being unchaste:
Surah 3:45-47. (And remember) when the angels said: O Mary! Lo! Allah giveth thee glad tidings of a word from him, whose name is the Messiah, Jesus, son of Mary, illustrious in the world and the Hereafter, and one of those brought near (unto Allah).
He will speak unto mankind in his cradle and in his manhood, and he is of the righteous. She said: My Lord! How can I have a child when no mortal hath touched me ? He said: So (it will be). Allah createth what He will. If He decreeth a thing, He saith unto it only: Be! and it is.
Besides, it is NOT true that the Koran denies a human guardian for Mary other than Jesus. The following verse actually implies that Mary was entrusted to a man that was chosen by the Jewish religious authorities in the Temple:
Surah 3:44 This is of the tidings of things hidden. We reveal it unto thee (Muhammad).
Thou wast not present with them when they threw their pens (to know) which of them should be the guardian of Mary, nor wast thou present with them when they quarrelled (thereupon).