Hera decided to get her revenge in one way or another, when realizing that Zeus had been with a mortal woman yet again. Now, Zeus made a fatal mistake the same day that Heracles was to be born, boasting that the descendant of him born that day would rule the race of Perseus. Hera knew that she could use this against Zeus and made him swear that it would be so.
She then went down to the earth and caused Menippe (The wife of Sthenelus, a descendant of Perseus and therefore also of Zeus) to give premature birth to Eurystheus. The birth of Heracles was prevented at the same time and he and his one night younger twin brother Iphicles were born too late.
Now, Eurystheus was to be king (in Tiryns), due to Zeus oath and Heracles destiny was open for mischief.
We do not know much about Heracles childhood and youth, at least not from myths that seems to be as old as the labours and so on. There is however one very wonderful story that tell us how Hera, in an attempt to kill Heracles while he still was young, sent two serpents to him. However the trick failed as Heracles managed to strangle the snakes despite his young age.
[...]the queen of gods in the kindling of her
anger sent presently the two snakes, and they when the doors were
opened went right on into the wide bedchamber, hasting to entwine the
children, that they should be a prey to their fierce teeth.
But the boy lifted up his head upright and was first to essay the
fight, seizing with inevitable grasp of both his hands the two
serpents by the necks, and time, as he strangled them, forced the
breath out of their monstrous forms.
Pind. Nem. I.40, Translation by E.Myers