I used to shower with the baby in the bath with me, get one of those sponge holder thingies then hes not going to be left anywhere (or shower when OH is there...?!) Though wherever you put him Im assuming you arent going to leave the two of them together unsupervised so there wont be any risk of her picking him up or whatever when you arent there.
Kids like playing with water, how about teaching her to put the plug in and just half fill the basin if she wants to play in the water whilst you are in the loo/doing something else so its not costing you money in water.
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