Math education techniques are a legitimate field of research worthy of publication. Science education is an academic field that publishes just like any other. Arxiv is a preprint server and not a journal - they don’t discriminate based on “impact” or interest to a certain audience. If you want that, read a journal that meets your desired criteria. It is perfectly reasonable to publish methods for educating children, or even results intended for an audience of children in Arxiv. A key purpose of a preprint server is to circumvent gatekeeping.