Of Ko Samui, Ko Chang, or Ko Lanta, which would you recommend most for a family with a toddler?

I think all three of the areas you've singled out (Samui, Lanta, Chang) can be great for families with a toddler. Two weeks is plenty of time so wherever you are, you could easily spend a week at the main island and then take one of the easily available island hopping boats (dry season only) to check out some of the smaller islands. I always recommend some of those smaller islands because the beaches are often more beautiful than on the main island.

Of the three areas I personally prefer the Koh Chang Archipelago and Koh Lanta / Krabi / Trang areas because, although Koh Chang and Koh Lanta are both sizable tourist destinations, they are both more laid back and not as developed or mainstream as Koh Samui.

Although Koh Chang does have some slightly seedy nightlife on the main road near White Sand Beach, neither Chang or Lanta are as seedy as parts of Samui, around Chaweng and Lamai. (That said, the south of Samui is a whole different story and you could easily stay there and never even stop at Chaweng or Lamai if you wanted.)

One thing that Samui, Chang and Lanta have in common – and I would also say this about Koh Phangan – is that they're all beautiful but also big enough to have totally different "vibes" or "scenes" in different areas. So on any of them you can stay in a quieter area that still puts you within relatively easy access of all attractions and other parts of the islands.

If you want to do some island hopping, a slowboat from Koh Chang to Koh Mak to Koh Kood, or ferry from Krabi town to Koh Lanta and then private longtail boat around the Trang islands, would be, I think, more exciting than the larger ferries to Koh Samui / Koh Phangan / Koh Tao.