While searching for one of Merlin's lost artifacts, Herc and the family become aware of just how active the babies have been.
When the location of the first artifact is discovered, the obstacles seem almost impossible to overcome in the short window available to them.
Of course, they never expected to be helped by four very active toddlers. The toddlers also thought it was great fun, to add to the family by magically helping their parents to conceive more playmates.
To further complicate things, they accelerated the births so they could all play together immediately.
All of this is done, while they were under attack, from an unexpected force that not only wanted to dominate both worlds, they planned to take control of the children, by taking over their powers, and their will.