Hi JustAdd. I do not live in Sri Lanka, but my 2 year old DD was born and brought up so far in southern India, so I'm guessing lots of things might be similar. I'm also sure that Sri Lanka might have differences, but I thought I'd share with you some things we've learnt.
One of the most useful things we bring is, suprisingly, ziplock bags (or similar!) These are suprsingly useful, and not available here.
We also use reusable nappies, as disposables do cause a rash (though we do use disposibles in AC and overnight). We have selected ones which dry easily, since in the rainy season drying can be an issue, though it's a breeze (so to speak) for the rest of the year.
Also, if you have preferences for things like nice nappy rash cream, kid's bath soap or special kids toothpaste, better bring them as only the standard Johnson Johnson kind of stuff is available here.
We also bring things like sulpherless dried apricots and ground almonds. These last for a long time and go a long way to making nutricious snacks for toddlers, since there's certainly not the same range of kids snacks which are available in Britain. Of course most dried fruits and lots of nuts are available here more cheaply, but I use the ground almonds to make biscuits with.
We also have a breadmaker. I know that might seem a rather large thing to put in your packing, but it's lovely having homemade bread, and really helps to make use feel at home. The local flour works fine, though we have brought our own yeast. We actually brought ours as carry-on baggage and the airline didn't complain, in fact they were rather interested! Certainly where we are, we can buy nice bread, but it's on the other side of town, and really not worth the hour and a half round trip!
We have found that the mosquito spray available here is rather chemically, so we sometimes ask people to bring us things like Mosiguard, or the boots natural mosi-spray, since we don't want to cover our DD in chemicals every single evening, and Mosiguard does seem to be effective enough to keep them off her.
We have also brought nice supportive sandals for her from England. It's not really necessary, since of course you can very well buy shoes out here, but at the same time, it's nice to know her feet are well supported, and it's almost impossible to get Clarks/Startrite quality stuff here.
We found it suprisingly difficult to buy good quality beach toys! Maybe you will have a different experience in Sri Lanka, but lots of the expats here have brought their beach kit in Ikea (sadly not us, and our flimsy local stuff is clearly not as good).
OK, well good luck with your preparations and packing, and I really hope that your life in Sri Lanka is as happy as my time in India has been!