You have to back up the older dog not least to let the pup know that you mean no when you say no but also to let her understand that all dogs arent play things. Monitor her behviour very closely and stopping it at the first sign will make things a lot easier, an old dog deserves a little peace and quiet especially if it not used to others.
Dont make seperating them exciting, be direct and firm, send her away - give her a warning command if she ignores you - which if she does, put her on a lead and have her sit, only let her off when shes calmed some and not before. I doubt whether distracting her will work that much as hes a new thing to explore and if your visiting you dont really want to be playing or moving about to keep her attention, you want to enjoy your family