Toys come to life at night, kind of like toy story, but with rules. While children are awake, their imagination is limited to their own minds, but at night, while they sleep, little bits of it escape from them while they dream. The power of this brings their toys to life. This means that all the toys act exactly the way that the kid would see them acting if they were real.
But the imagination that escapes is sometimes tainted when they're having a bad dream, and brings their nightmares to life. Nightmares, as they are called, are creatures made of negative emotions and imagination. Their goal is simple, fuel those bad emotions in the child's sleep so they create more nightmares, bigger nightmares, stronger nightmares.
And the stronger the nightmares get, the weaker the toys become. So, for the safety of the kid and themselves, they must defeat the nightmares as they come every night, and keep the kid happy.
A major plotpoint is that the kid is building a complex robot model kit, which, when completed, will be the deus ex machina that saves the night at the very end when all hope seems lost.
Working title for this is: The Guardians of Bedtime.