Whatever your father is paying for his hotel entitles him to have a certain number of guests on his reservation, and those guests are entitled to early entry.
If he can add all of you as his guests, you can do this.
People will debate the ethics of this. Does it go against the spirit of the Early Entry rules? Maybe? At the same time, he's paying for it and adding up to x number of people to the room reservation is allowed.
I can see where people consider this to be a grey area, but I personally think it's fine.