Just came back from Sri-lanka last week for a month's holiday as my dad lives there
Pampers are slight more expensive than over here but much lower quality - quite plasticky and seemed to be smaller even for the same weight (of child).
I took out pampers and cow and gate milk for DS (15months).
The only beach near colombo is mount lavinia, which is okay if you cannot leave colombo, but beaches "down south" bentota onwards are amazing.
Arpico - big supermarket has a good stock of baby products, and right next door is a sort of a food court - Urban kitchen with an airconditioned children's play area. Also near by in Victoria Park has a sort of fixed children's funfair Sathitui Uyanaya.
ALso, be aware of the seasons. DEcember, Jan Feb are lovely for the west coast. Gets very hot in april, so a lot of people go to Nuwara Eliya (tea esates, mountains - very british!) Also now very trendy is the East coast. Arugam bay is a popular surfing area, especially with the younger crowd in april.
They are spending a lot of money on infrastrucutre. The new highway means you can drive to galle in 1 hour as opposed to 3.5hrs and is opening up the east of the island which is (currently) gorgeous and unspoilt.
Also they are building a new express train service to galle from colombo, will be open by the end of the year.
Sri-lanka doesn't have all the conveniences of the west, but the people love kids - my DS seemed to have a constant entourage, so your dd will have an amazing time!