Wa alaikum salaam wa rahmatulahi wa baraktuh dear sister,
Toddlers of this age become very experimental with everything as they explore the world and learn through play. This will usually include exploring their body also. Given that her genitals will be covered u the majority of the time, they become more of a mystery to her than any other part of her body, so naturally, it might seem that she is taking an unhealthy interest in them at times.
What you choose to all them is a matter of choice, each family will use different words to describe them. Private parts could be a good choice of words because it describes exactly what they are and will instill in her from a very young age, that these parts of her body are private, and private means that no one should see them or have anything to do with them. Having this attitude from a young age will help to teach her about modestly and covering up for the reasons that Allah has prescribed to us as women.
Using the term private as well may encourage her to also not spend too much time exploring her genitals too, because, whilst children of this age will explore, and need to do so in order to learn about the world, you also have to be careful not making it too ‘normal’ to do so that she feels it is appropriate to do this regularly. If she does continue to explore this body parts in a way that you feel is becoming unhealthy or too much then it is important that you address it with her in an appropriate way for her age.
At her age, she won’t see any wrong in what she is doing and therefore scolding her for doing so might lead to some confusion. What you can do however is gently distract her with something that she enjoys to do so she is diverted from what she is doing without making a big fuss out of it. Obviously, on top of this, her genitals should be covered at all times anyway which will also reinforce the fact that these are her private parts and they are not for anyone else to see.
May Allah reward you for your good intentions in trying to raise you daughter in the best way.
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