I've observed that when someone wants to say 'poor thing', they say something like 'kawaii sonna' and I know that kawaii means cute. Can someone please explain? Thanks.
You are mixing i-adjective かわいい (kawaii, "cute, lovely") with na-adjective かわいそう (kawaisō, "poor, pitiful"). These are simply different, although they share the same etymology. かわいい（かはゆし） actually meant 'pitiful' in old Japanese, but there was a shift in meaning many years ago.
We say おいしそう (oishi-sō, "looks yummy"), たのしそう (tanoshi-sō, "looks amusing"), etc., but we don't say かわいそう to mean "looks cute", because it's confusing. Basically whenever you hear かわいそう (kawaisō), that must mean "poor".
For more information, please refer to this question.
Well, I'm not an expert on Japanese, but as a Chinese who is learning Japanese, I'd like to offer something from this perspective.
The dictionary says かわいい can be written as 可愛い while かわいそう can be written as 可哀相. For a Chinese this seems quite straight-forward. You see, in Chinese, 愛 means "love" and 哀 means "pity", and these two characters have the same pronunciation "ai", except for a difference in tone. Since Japanese doesn't have the tone system of Chinese, it is natural that they are pronounced identically in Japanese.
By the way, 可/か means "-able", therefore 可愛 is "lovable, cute" and 可哀 is "pitiable, poor". 相/そう means "look, appearance".
These two words, in the eye of Chinese people, totally look like loan words / calque / words created by Japanese people using Chinese word roots. But again, I'm not an expert in Japanese etymology. They and their Chinese counterparts certainly could be just false friends.
To be specific, what sounds like "Kawaii sonna" is probably acually "kawaisou na". Kawaisou, as explained, means pitiable and na is similar to ne, but is more informal and gives the sentence slightly more of an exclamatory feel.
Compare "atsui na", frequently used in hot weather, and meaning essentially "Gosh, it's hot isn't it".