Yeah that sounds like a mess and I admire your willingness to intercede for the sake of the child. I would say that if you are initially invited into the house by the residents, and subsequently attacked to the point where you would need to defend yourself, you would be justified in doing so. The castle doctrine addresses "unlawfully" entering or remaining in the dwelling. If you are asked to leave do so immediately, especially if armed, because if you remain beyond that point in time you are trespassing "while armed" if you are "packing". My suggestion would be to call law enforcement or DCF immediately if you fear for the well being of the child. Be a good witness and only involve yourself "hands on" if there is a clear and immediate threat to the child's life. Deadly force is only authorized by statute if there is an immediate likely hood that what ever is happening to the victim will cause death or great bodily harm.
I'm not a lawyer and this is not to be considered to be legal advise. This is my individual opinion of Florida statutes