Praise be to Allaah.
Everyone who possesses wealth that reaches the minimum threshold (nisaab) and one year has passed since he acquired it, is obliged to pay zakaah on it, whether he is old or young, of sound mind or otherwise.
No differentiation is made between orphans and others in this case, because zakaah is what is due from wealth, as Allaah says (interpretation of the meaning):
“Take Sadaqah (alms) from their wealth in order to purify them and sanctify them with it”
In Fataawa al-Lajnah al-Daa’imah (14/293) it says: It is obligatory for the guardian (wali) to pay the obligatory zakaah on the wealth of minors if it reaches the nisaab and one year has passed since it was acquired. End quote.
This has been discussed in detail in the answer to question no. 75307.
Based on that:
1.If the wealth of each of these orphans has reached the nisaab as defined in sharee‘ah (the value of 595 grams of silver), then you have to pay zakaah on his behalf.
2.With regard to the profits that you put in their account, if it came from your capital then it is a gift from you to them, and it comes under the same heading as wealth acquired during the year. So you should work out a separate year for that, starting from the date you gave it to them.
see the answer to question no. 93414.
3.With regard to the profits that come from their own capital, they should not work out a separate year for that; rather the year for them starts from the date the capital was acquired and zakaah should be paid on (the profits) at the same time.
One of the duties of the wasiy (guardian) is to take care of orphans placed under his care, look after them, protect their wealth and invest it according to what is in their best interests according to sharee‘ah. There is no doubt that what you are doing, using your money for trading in shares and putting the profits in their account, is a good and righteous deed for which you will be rewarded.
And Allah knows best.