Go above their heads. Hire your own independent (unbiased) inspector.

I agree with Curt. You have given the builder time to respond and the fact that they haven't doesn't instill confidence. At this point, I would want the person who analyzes that crack to be working for me rather than the builder. Hire a home inspector with a great reputation. It shouldn't be very expensive in the grand scheme of things and even if there is nothing major wrong, the peace of mind it will give you will be worth every dime.

The builder said they will seal the crack before the floors are installed. I don't think that's enough of a response.

I would want to know that the slab is structurally sound before installing a floor over it. If you are uncomfortable with the builder's response, hire an inspector for a second opinion and don't feel bad about it. Caveat emptor - latin for "let the buyer beware".

This is a very new construction. Technically the slab is still curing (it cures 1mm per day) and can continue to cure for another month or two depending on thickness, air temperature and humidity. Large blocks of concrete crack - and there is no way to prevent this (there is but NO ONE wants an expansion joint every 5 feet). Concrete cracks as it dries. You are two months into a cure process that COULD be another month before it finishes.

In essence, slabs cure at 1 month for every inch of thickness. Not knowing how thick your slab is or if it is "sinking" in one area or heaving in another, I can't say if this is structural or not. If your contractor states it will be "sealed" before a floor is laid, then the answer to the problem is already at hand.

The curing may not have finished. Until then, nothing can be done about it. The sealant has to go down AFTER the cure is finished. That requires moisture readings around about the time the cure should finish. At this time corrections can be made, cracks filled and sealant used.

Feel free to take photos. Do this every few weeks. If you feel you need to lay a straight edge with a level on it, do so and photograph it if things are not flat. Talk to concrete specialists in your area to find out what is "normal" around you. If 60-90% of your local slabs crack like this...then you have experienced a local norm. If less than 5% do this, then there might be a situation underneath the slab that has caused some issues. Even the best laid plans can go astray when an undiscovered puddle of water exists a few feet below the slab.