Then she pointed to him. They said, "How can we talk to one who is in the cradle, a young boy?" (In the cradle, idiomatically means a young person. Mary and Joseph had stayed in Bethlehem for several years)
Thereupon she pointed to him (- her son Jesus meaning thereby that he will answer them). They said, `How should we speak to one who was (till recently) a child in the cradle?
As Mary had vowed fast she only pointed to the child (suggesting they should ask him)... They said, “How can we talk to a baby in a cradle!”
Sayyed Abbas Sadr-Ameli
Then Mary pointed to him. They said: 'How shall we speak to one who is (yet) a child in the cradle?'
Al-muntakhab fi tafsir al-Qur'an al-Karim
She simply pointed her finger at him -the babe- with an intimation, as it were, that they ask or speak to him. But they exclaimed: "How in Allah's Name can we discourse with a new born babe?"