In the name of God, Most Gracious, Most Merciful
1) To Loki: I would like to concentrate first on the Arabic Gospel of Infancy. We can look at your other 'source' later.
2) Tisdall said the Arabic Gospel "so bad that it is hardly possible to believe that it dates from Muhammad's time." How do you interprete that? I read it to mean that it could not have been translated during Muhammd's time. The period of the Quran revelation was the time when the Arabic language was at its prime and the Arabic Gospel could not have been at that level. It is interesting to note when was the first Arabic Gospel (Gnostic or not) translated from Coptic. Search for it.
3) Being an anti-Islam, Tisdall further proposed that it could be from a Coptic source and said (without proof) that Mary the Copt could have taught or told the story to the Prophet.
4) Hence, Tisdall ran away from making a 'scholarly' statemnet that the Arabic Gospel was available during the Prophet's time and instead focused his attack on a supposedly story-teller 'Mary the Copt'.
5) Now, did the story in the Quran of Jesus speaking in the cradle came from Mary the Copt?
The verse concerning Jesus speaking in the cradle is a Makkan verse. Mary-the Copt was sent to the Prophet in the seventh year of Hijrah and by that time this verse was already revealed!
6) There is No Proof that the Prophet copied from the Gnostic or Jewish sources.
salam