The Baby Taj (Itimad-ud-Daulah) was built several years before the Taj Mahal was started.

Before the Taj Mahal was built, its design was already standing in the form of Itmad Ud Daulah's Tomb. Built to represent a silver jewel box in marble, this mausoleum houses the body of Mirza Ghiyas Beg, the father of the Mughal Empress Noor Jahan. After Noor Jahan married the Mughal Emperor Jahangir, Mirza Ghiyas Beg was appointed the Lord Treasurer of the Empire or Itimad-ud-Daulah.
After her father's death, Noor Jehan built this mausoleum for him from 1622-1625 A.D. The first example of a tomb built on a riverbank in India, Itmad Ud Daulah's Tomb situated on the banks of the Yamuna, has many design features that were later used in the Taj Mahal. Itmad Ud Daulah's Tomb marks a significant departure from the tombs of the Mughal dynasty built earlier, because this tomb follows the central Asian pattern of a domed structure, set in a formal garden, with waterways and paths laid out in a geometrical pattern, a design followed while constructing the beautiful Taj Mahal.
It's gardens swarm with monkeys.