The longest night in Egypt / David Anthony Durham.

Enhanced descriptions from Syndetics:

The Shadow Prince saga continues as Ash and his heroic friends rush to save Ra from the belly of Apep and bring light back to an Egypt that has been plunged into darkness.

Back at his evil tricks, Set, the devious god of chaos has rid Egypt of the sun god Ra and plunged the kingdom into never-ending night. He's even managed to trap the rest of the gods in a palace chamber without the use of their magic. Now demons run amok in the city, filling it with chaos and destruction.

But hope is not lost! Ash, Prince Khufu, Seret, Gilli, and two new friends, Thea and Iset, must brave the depths of the Duat--the spooky, cavernous Egyptian underworld--to rescue Lord Ra. Numerous demons fly about them in the darkness. Deadly peril awaits them around every craggy corner. They must fight, trick, sneak, and solve riddles to pass through each successive gate. If Team Shadow Prince can manage to get through all that, they will still have to face a demon like none they've seen before: the fierce, wily, enormous, god-eating serpent, Apep.

Will the kids prevail? Can Ash and his friends reunite Ra's magic with the sun and bring power and light back to the kingdom? Or will Egypt remain in darkness forever?