In Cantonese, what is a less "childish" way to call one's own parents?
In Cantonese, 我爸爸 and 我媽媽 are perfectly fine to use to refer to your parents, no matter what your age is. They are neither rigid nor childish. Slightly more familiar terms are 我阿爸 and 我阿媽. In a very formal context, You can also say 我父母.

To address them directly, most people use 爸爸 and 媽媽, or 阿爸 and 阿媽, depending on which their family prefers.

父親 and 母親 are mainly for writing.