How you met: your mom was pregnant during the attack on Serbia. Pietro helped you come out. You were the only survivor of your family. Pietro held you close with him at all times until they were on the helicarrier. He reluctantly gave you to the medical team. They allowed you in his custody until something could be arranged. Pietro ended up taking you home.

Meeting the others: obviously the heroes made one liners at you as they sat dazed in the helicarrier, high on adrenaline. After everything settled, Wanda came to see Pietro and was the first to meet you. Eventually all of the heroes had come to visit one point or the other. They agreed you were cute.

Bonding: Pietro was with you from literally day one of your life. Deeeep, deep deep down he felt some guilt for having a hand in the Ultron fiasco, and in moments of emotion, he promised to keep you safe from every harm. Given his charming personality, he fawned over you very much, spoiling you.

Sickness: Pietro's usually nonchalant about life, more like a fun uncle. When you get sick or show any other baby distresses, he panics for a second before using his speed to read every care book he can find. If it's anything more serious, Wanda usually steps in as Pietro worriedly looks over her shoulder. Regardless of whoever takes care of you, you usually only fall asleep in Pietro's arms.

Powers: your biofather was apparently a winter soldier who had escaped the facility due to an error. With no handler, he was able to live a normal life. You inherited his superhuman health.

Danger: Pietro was Pietro. You got into a few dips and dings here and there. Mostly however, he used his speed to catch you in time or baby proof something just before you got to it. As you grew older, you inherited your dad's... curious personality and often got where you shouldn't be. Luckily, you had your accelerated healing.

Kidnapped: there had been rumors of a super soldier baby, and eventually bad guys found you out. They abducted you from daycare. Pietro reached out to Cap and Bucky to track down the people who had taken you. Eventually, Quicksilver, with Captain America, Winter Soldier, Scarlett Witch (and Magneto) came to your rescue. You and your father stayed with Magneto after, waiting until team Cap made sure no one would come after you again.