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, we thank you for being keen on working for the sake of Allaah and His Religion, and we ask Him to reward you and enable you to achieve this target.
You should know that having children is a right of both spouses, so it is not permissible for you to refrain from having children without the permission of your wife. This is if we presume that you are really in necessity for temporary contraception, but we think that you are not at all in need for it. The existence of the children is not an obstacle from performing Da’wah or doing good to people. The righteous predecessors used to have many children and that did not prevent them from practicing Da’wah, enjoining good and forbidding evil, and even making Jihaad (fighting in the Cause of Allaa). On the contrary, Allaah may bless you with children who would do all this and you will get a great reward.
Look at what the Prophet Sulaymaan (Solomon) said when he was keen to have more children; the Prophet said: ''Sulaymaan ibn Daawood (David) said: “I will go around (have sexual intercourse with) seventy wives of mine, and each of them will conceive a warrior who would fight in the Cause of Allaah.” [Al-Bukhari and Muslim]
In any case, we advise you not to consider having children as an obstacle from practicing Da’wah.
Finally, you should try to solve the problem which you mentioned in the question and which is your weakness in organizing your time as this is the real obstacle for you. For more benefit, please refer to Fataawa 90092 and 84086.
Allaah Knows best.