Children like to experiment, and it is common to try to use the bathroom like the parent or older siblings of the opposite sex. Toddlers are barely beginning to comprehend the differences between boys and girls. Since they learn by doing, they need to go through the motions. Watching and talking are not enough to satisfy their curiosity.
Little boys may want to use toilet paper after they urinate because Mom does. Toilet paper is fun to use and doesn't hurt anything, so let them.
Little boys may want to stand up to urinate because Dad does. Even though their aim is poor, let them stand and show them how to push their penis down so they can give it a try. Either sitting or standing is perfectly acceptable for a tyke, though sitting is far less messy.
Some little girls want to stand up to urinate because Dad does. Lift your daughter so that she is standing up with one foot on either side of the toilet seat, keeping your hands on her waist so she doesn't fall. Encourage her to bend her knees slightly so she doesn't make a mess.
Some children like to sit facing the tank, and it becomes a habit. There is nothing wrong with sitting backwards! Indulge your child.
End messy experiments by saying, “Mommies and little girls/boys sit. Big boys and daddies stand up to urinate and sit to have a B.M.” However, until children's curiosity is satisfied, they will probably conduct their experiments in secret, which can be far messier. It is safer — and neater — to guide them.