Not trying to poo poo your great idea, but Sri Lanka was really the only place I have been that I haven’t thoroughly enjoyed. My wife and I have done lots of independent travelling very much like you have described (Asia, India, S.America) and we really didn’t enjoy Sri Lanka – while it has a lot it offer it is really quite difficult to go independent, very hard not to get swept into semi organised deals, really hard to describe why we didn’t like it….I will have a think! And I think a 12 & 15 year old would struggle for activities/stimulation.
But I would HIGHLY recommend Vietnam and Cambodia. A few days cycling around Ankor was a lifetime high. P
(Edit – we did Sri Lanka in mid 2011)