When HYDRA sends The Winter Soldier to bring them the four-year-old Boy Who Lived, the outcome is not what they expected.

The Avengers notice Tony acting weird for over a year but are unable to do anything about it. He seems more closed off, jittery and he's always checking his phone. That is, until he gets a call from someone he's been keeping a secret, a secret son they are about to be introduced to in the worst way.

Harry is being held by Hydra. The Maximoff twins are being held on the same base as him. Pre/beginning of Age of Ultron. I may have fudged the movieverse timeline a little.