How is momentum conserved when the ball collides with the floor?
Momentum is always conserved, irrespective of the size of colliding bodies.
In a two body system, let
Now momentum is defined as
The total initial momentum
And final momentum after the collision is
By conservation of momentum
In the given problem, the two bodies are 1. a ball and 2. floor. As the floor is rigidly connected to the building, standing on the earth's surface it is appropriate to assume that the second colliding body is earth. Also that in a frame reference to earth it is the ball which moves and earth is at rest. The expression reduces to
For simplicity and for sake of argument assuming that the ball bounces back elastically in the reverse direction after collision,
Inserting this in the equation we obtain
We know that mass of earth is
For a bowling ball for kids the ratio
This expression has been derived for a special case.
Generalizing, it can be seen that after the collision speed of earth is very very small quantity because of the ratio of masses of the ball an earth being present in the equation. We are not able to measure such a quantity with respect to speed of earth in its orbit around the Sun which is
Having said, we have no reason to believe that the momentum is not conserved.