Praise be to Allah.
It is not permissible to hang up pictures at all, whether of children or adults. That is because of the stern prohibition narrated from the Prophet (peace and blessings of Allaah be upon him) against keeping pictures and his command to ‘Ali (may Allaah be pleased with him): “Do not leave any image without erasing it or any raised grave without leveling it.” Narrated by Muslim, 1/66
Hence pictures must be removed, erased or burned and not kept.
Hanging up pictures of animate beings inside houses or elsewhere deprives the people of that place of a great deal of good, which is the angels’ entering that house. It was narrated from Abu Sa’eed al-Khudri (may Allaah have mercy on him) that the Prophet (peace and blessings of Allaah be upon him) said: “The angels do not enter a house in which there are statues or images.” Narrated by Imam Ahmad; Saheeh al-Jaami’, 1961.
It is permissible to hang up pictures of inanimate things such as trees, mountains, seas and natural views, or other inanimate drawings, without being extravagant.