Hi Shawna,
Great choice, the
Beach Club is one of my favorite Resorts at Disney World!
I'm not sure if this is an option for you, but I'm curious if your Dad can share a queen bed with your son? Your daughter could then sleep on the day bed. While it is technically about the width of a twin bed, it may be a bit small for your Dad (depending on his size). He'd likely sleep more comfortably in the bigger bed.
If your family is like mine, you won't spend much time in the Resort room. We go there just to sleep and shower. If this is the case for you, I think you'll be fine with all 5 of you in the room.
In my opinion, the priceless memories that you will make with your Dad while on vacation will far outweigh any minor inconveniences of having an extra person in the room. My theory is: The more the merrier!
Hope this helps and that you have a Magical vacation with your family,
Kaylene