Are you trying to get the balls to interact with the fluid or are you just happy having the ball go into the fluid? If the latter I would just double check that your domain is just inside the mesh at the bottom of the cup and not out past it as the fluid will only interact with the domain area. So if you do the domain box have the bottom of the domain box about mid way between the bottom verts of the exterior of the cup and the interior verts of the cup. Also you can have a solid object in the shape of the interior of the cup that fills to the depth you want the fluid and place it in the cup to the size where it is closest to the interior of the cup and set it to fluid then bake the sim again and once it is baked you can move that object outside of the rendered layers as it is no longer needed just don’t delete it in case you need to adjust it some more.
Another tip is once the sim is baked you can now adjust the domain size to adjust for any portion of the simulation that is showing outside the mesh and it will still play as it did after the bake. Simulations are not always an exact science and need some fiddling around with to make them work and look right. You can also apply a subdivision surface modifier to the simulation after the baking process to help smooth out the liquid more without having to raise the final resolution.
So essentially you will have three parts to the simulation as follows, the domain cube set to size and physics set to fluid domain, the cup physics set to fluid partial slip, and the fluid object in the cup physics set to fluid.