I'm afraid my own experience of that is very far in the past, and, generally speaking, Hogwarts students do not require this sort of training.

Mind you, there are potions that can help if she really has difficulty discerning when she needs to urinate or if she develops a problem with bedwetting. (I keep a supply of Eupatoria Pliny's Dry Dreams on hand here because I do have frequent need of it, especially at start of term with the first-years.)

The one other tip I could offer is to use a quick colouration charm to turn the water a vivid blue, so Bea may enjoy turning the colour green when she adds her own yellow-coloured fluid to the blue. My Rue found that a most engaging game when it was time for her toilet training to begin.