After poking around a bit in the Shuri-Naha Dialect Dictionary that covers the Okinawan branch of Ryūkyūan, it appears that here, at least, labiovelars did persist. A sampling of entries that show maintained labiovelars:
クヮンジミ, Japanese 缶詰【かんづめ】
クェーチウイウェー, Japanese 快気【かいき】祝【いわ】い
クヮンヌン, Japanese 観音【かんのん】
カングヮーシ, apparently equivalent to Japanese 羊羹【ようかん】 + ...
Some preliminary research:
日本国語大辞典 has an example sentence dating from 1813: 「ばばちゃんいやいやと云ってだだをいふだ」. Dictionaries do make clear that it came from さん, but none of them list it as 幼児語 (baby talk). One reason this might be is that it has become a standardized way of addressing young children and female friends, so it is no longer considered 幼児語 as everyone uses ...