If me and boyfriend both have sickle cell trait, how will we prevent our children from having full sickle cell anemia?

Nothing. Unfortunately, there would be 25% chance for your child to have sickle cell disease and 50% chance for sickle cell trait . With our technology now, nothing can alter the risk. Sorry to say , the only thing for somebody with sickle cell trait to completely prevent any possibility of having a biological child with sickle cell disease is by having a child with somebody with normal hb.