In some cultures, if you have a belly button that looks like a hole, your belly button is thought to be beautiful, but if it protrudes, it is not beautiful.
Is it correct to say "Her belly button sticks in/out" in everyday English?
You could use various levels of formality:
To a small child, or an adult to whom you don't mind talking childishly
You have an outie, and your sister has an inny
My son has a protruding navel (or a navel that pokes out) and my daughter has a sunken one (or one that is sunk in or depressed)
In formal or medical writing
The male child's navel is everted, and the female sibling's is inverted