The reason I asked is that in some states, a tenant can place rent in an escrow account (handled by the court system) until an issue can be resolved. In the case of repairs, this temporary loss of rent often "lights a fire" under the landlord to get the repair done.
Other states allow renters to address the repairs (when their landlord has not responded) and bill the landlord or withhold the equivalent amount from their rent.
If the plywood (and hopefully this is cement board for a wet area like a shower) is up around the tub area, the next step is tiling or adding in the bathtub insert. Tiling would take longer since it's a slower process and grout must set for a certain period of time before the area can be used.
However, to be honest, we were able to use our bathtub when we remodeled our bathroom and before the tile was up. We just had to be careful not to splash water on the untiled walls.
At any rate, you have the right to request to your landlord (in writing so you have documentation) that the work on your one bathroom be completed soon so you have access to this area. You also can request that he/she let you know how much longer this process will take as the current status of an incomplete bathroom is certainly an inconvenience for you and your family.
I'm assuming you have access to the toilet in the bathroom, correct? A landlord must provide a living space that includes sanitary facilities (meaning a toilet).
This consumer advocacy service for your state may help..
http://www.thelpa.com/lpa/landlord-tenant-law/south-carolina-landlord-tenant-law.html