The context to this is a story I'm reading. A high school girl says she was assigned to make a poster for class that said "たいようさんさん". Her friend, in response, seemed confused and mused, saying he "wonders if the teacher is quite old." The girl then gets angry and states she's the one who came up with it, not the teacher.
With that, I'm confused on what たいようさんさん means. At first, I thought it simply meant "Sparkling sun". But her friend's response seemed to indicate that something about it was old-fashioned somehow. I began to look to see if さんさん was some older equivalent to さま, or if たいようさんさん/太陽さんさん/etc. was in reference to some old show or song. I wasn't able to find any consistent results, save for this magical girl anime called "Smile PreCure!" but this story was written in the early 2000s, a whole decade before that anime came out.
Maybe the answer is simple, or I'm overthinking it, but I'm having trouble figuring this out. Especially since the way it is written has no kanji, it makes it slightly more difficult for me to figure out.