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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
May Allaah give you the best reward for your concern for those orphans. Of a surety, you have a reward with Allaah The Almighty, according to your sincere intention and good objective if you marry their mother to be able to look after them. However, her being a widow with orphans does not make it due on you to marry her, for marriage becomes due in certain cases according to Islamic scholars, and this case is not one of them. This is because it is possible for you to look after the orphans without having to marry their mother. You should know that jealousy is an innate characteristic of women which they cannot avoid. The wives of the Prophet, sallallaahu ‘alayhi wa sallam, used to be jealous for him.
For this reason, if you decide to marry this woman, you should know that your first wife would naturally become jealous for you, and stand against this marriage and do her best to prevent you from it. Therefore, you have to handle such circumstances with wisdom and Sharee‘ah-defined guidance. You should assure your wife that your marriage to another woman would not undermine her rights. You can also encourage her to seek the reward from Allaah The Almighty if she helps you do this good deed which you intend to do.
Concerning your question about your reward in this life and in the Hereafter, the answer is that you would receive a reward in accordance with your sincere intention and good objective as we mentioned above.
It is narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "I and the one who takes care of an orphan would be [as close to each other] as these in Paradise," and pointed with his index and middle fingers. [Al-Bukhari and others]
It is further narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "The best Muslim household is that in which there is an orphan to whom good is done." [Ibn Maajah]
The Prophet, sallallaahu ‘alayhi wa sallam, also said: "The dearest of your families to Allaah The Almighty is that in which there is an orphan who is treated kindly.”
Allaah Knows best.