I had an idea for something like a light novel of some sort, but I have a creeping concern. If I published a fantasy book series, each book has 100 pages each, would it be considered a children's book for being too short?
At 100 pages, they would not necessarily be considered books, but novellas. I do not believe length alone would place it as a children's series; it is up more to your writing and content. Sometimes, despite author's and publisher's intentions, (and despite content or length) books can end up being shelves wrong- I believe this has happened to Jay Kristoff with his Nevernight Chronicles,which he has repeatedly expressed contains adult content, and yet it gets shelved as YA because he often writes YA. Victoria Shwab deflected mistakes like this by changing her name- Victoria Shwab for her children's/YA books, and V.E. Shwab for her adult books.
For reference closer to your own project, I think you could look at Seanan McGuire's series, Wayward Children. They are very short novellas which do contain adult content, and are often shelved with adult fantasy, but it does happen sometimes that it is shelved as YA/ Children's.
It is not so much down to legnth as just mis-coordination within the industry, I believe.
Length doesn't determine categorization, it's the other way around. Figure out your target audience first, and then determine ideal word count. Children's books are generally shorter than adult's books, but an adult book is not just a long kids' book, there are differences in vocabulary, tone, themes and content.
In any case, however, it is never pages that are counted, but always words. Adult novels should be at least 50k words and probably closer to 60k. Children's books can be much shorter, depending on how young the audience is.
If your book is better targeted at adults, but shorter than 50k, package two or three of your installments together and sell them as one book.