Like Pia2am, I would not recommend volunteering for such a short period of time. I have done this somewhat naively and although I had a fantastic experience and loved every minute, I have come away and after some thought afterwards decided that my stay made little impact and probably was actually more detrimental than helpful (kids have to deal with strangers coming and going every few weeks, attachment etc). If I were to volunteer again, I would make sure I had a good long time to be there and really make a difference, like some of the others who I met whilst I was away.
However, if your mind is really set on volunteering, I can suggest Original Volunteers. They were great with me and my friend and made our whole trip really good. We went to Ghana and the people there were lovely, kind, helpful people who wanted the very best for everyone involved.
If you are keen to volunteer, could I maybe suggest you do a more environmental project that working with school children/orphans etc? You can make a difference and not have a potential detrimental effect on the children at the same time.
Have a great trip whatever you decide to do!!