بسم الله الرحمن الرحيم
(Fatwa: 1006/763/D=1434)
The Muslim children who die before they attain the age of puberty shall enter into paradise by the special grace of Allah and they shall intercede for their parents, in-shaAllah. There is no harm in doing isal-e-sawab for them as it shall raise their status as well as the person who is doing it shall benefit from this act. Read the following dua extensively:
انا لله وانا اليه راجعون ان لله ما اعطى وله ما اخذ.
Also think that it was a trust of Allah and He kept it with you as long as He wished and took it back when He wished. And if we have patience over the grief we suffer then Allah shall grant us uncountable reward merely by His grace. Also think that Allah is the Ruler and Hakim (All-Wise); hence no work of Allah is free from wisdom, rather these issues involve uncountable wisdoms and He is the Ruler and is free to do what He wishes, none has the right to object to Him.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband