I have to tell you there is no time when a boy and girl will share a bathroom. Please post some to scale foor plans .

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I would leave it as is.

Unless this is the only tub in the house? Forget the tub/ shower.

Two genders can USE a bath. Not at the same time unless pre k age:)

The "buddy" bath really isn't. Someone get up earlier

You might invest in some graph paper to make it easier to draw your floor plan to scale and to read it as well.

If the bathroom has one door from a hallway, add only one sink, preferably centered, with two separate his/her cabinets above and/or below, with the center base cabinet shared -- toilet tissue, etc.

Are you familiar with jack&jill bathrooms between two bedrooms? If the bath is between two bedrooms and you can create a separate powder room -- toilet and sink -- for each with a separate room in between those two powder rooms rooms for a shared tub or, depending upon the age of the children, a shared shower, you might consider that.

You have to begin with a very accurately drawn plan with every dimension as it exists now. Show every window, door, with their dimension.

If its going to be gutted? Your best bet is a CKD ( certified kitchen and bath designer) . At the very least, a good interior designer, with appropriate trades at her finger tips. The money you save in regrets will be your own.

I think I'd leave it as is, but maybe remove the physical door -- not sure I'd want kids to handle a door handle before they watch their hands! Could allow some sharing in a pinch, but agree it's mostly unlikely.