you may wish to visit the ncss website, and the MCYS website too, not
to mention CDAC, SINDA, Mendaki.
alternatively, you can always drop by the Family Service Centre at
your neighbourhood, or your CDC, to find out how you can volunteer
in the area of your interest.
for the target group you mentioned, there are really many ways to
volunteer, and it is possible to go down to greater detail too -
children who have terminal illnesses, children who live in poverty,
children who are at-risk, children who have committed crimes,
etc.
good luck! 