How to answer to a 7 years old the hijab for women not for man?
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, it is important to mention that dialoguing with children and responding to what they ask for should be done in a way that they understand. A wise educator should listen to the question of the child and if the answer to the question is important and beneficial to him, then he should answer him according to the question and according to the understanding and age of the child. However, if the answer is not important for the child, the educator may answer him in a beneficial manner; this is indeed what is called the wise manner (in educating and cultivating).
For instance if the child asks about the reason why women were Hijab and not men, and he is too young to understand the answer, then one may answer him by saying that it is Allaah Who wants this. Allaah wants women to wear Hijab and He does not want men to wear Hijab, and we are ordered to obey what Allaah says and what the Prophet says, and that whoever obeys Allaah and His Messenger, will enter Paradise.
The parent may instruct and guide his child using any explanation by which he makes him bound to the Order of Allaah and that of His Prophet and raise in him the feeling of being attached to Allaah, seeking His reward, and that it is Allaah who orders and forbids, and He should be obeyed.
For more benefit, please refer to Fataawa 88490 and 87142.
Allaah Knows best.
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