A tub is likely part of what you rented, and removing it without replacing it in a reasonable amount of time changes the premises from what you rented.
On the other hand, it would not be necessary that the removal and replacement be simultaneous.
There are two standards:
1. What is the value of the rental unit? Removal of a bathtub would reduce the value of the rental unit. You could renegotiate the rent or perhaps get a discount for the time the bathtub is gone.
2. Is the unit habitable? You would have an argument if you were not able to bathe, but it sounds like you have a shower. Special issues like your daughter would not matter unless outlined specifically in the ADA. Your unit is habitable, so you cannot stop paying rent.