"Pushed away from you" means "pushed away from you"
A wave of thunderous force sweeps out from you. Each creature in a 15-foot cube originating from you must make a Constitution saving throw. On a failed save, a creature takes 2d8 thunder damage and is pushed 10 feet away from you. On a successful save, the creature takes half as much damage and isn't pushed.
—Thunderwave, Player's Handbook, pg. 282
Although 5th Edition has rules about how to play the game using a 2d grid, the combat mechanics are presumed to occur in the Theater of the Mind. There's no reason to believe that a spell which "pushes a creature away from you" must require that force to be parallel to the ground, or restricted to any specific direction, unless the spell says so, which Thunderwave does not.
So amending your scenario a bit: if you were crouched down, or otherwise elevated below your targets and then cast this spell, your targets would be propelled 10 feet into the air, in the same direction as the direction from you to them, meaning they would have to be propelled a few feet into the air, or if they were directly above you, exactly 10 feet in the air.
Now, there's a lot of [valid] reasons that many DMs will abstract this: only in the scenario where someone is directly above you would they fly a full 10 feet above the ground, and any distance less than that they would fall harmlessly to the ground, since you only take fall damage if you fall at least 10 feet. Many DMs will therefore simply push them back 10 feet regardless of the angle—or they will apply some basic trigonometry and push them back the appropriate horizontal distance without even considering their overall vertical travel. Either way, it's rarely productive to hash out those precise details.
So in practice, you may find many DMs simply call it 10 feet of horizontal distance that targets get pushed back, regardless of the angle of incident. But in principle (i.e. by Rules-As-Written), creatures can and should get pushed into the air if the angle from which they are struck by Thunderwave suggests they should.