
I mean I suppose it is weird, but in real life I've heard many children say it. Kids don't see it as a weird thing, they see it as a promise to be close forever, and that's it, nothing more, nothing less. They don't see it as something only romantically involved people can have. Most authors use it as a wholesome thing, although it's understandable why people find it weird, but I think you just have to understand that the author probably didn't mean anything weird

Yeah it's a disgusting trope in anime. But it's not uncommon for younger children wanting to marry you... they don't understand what that even means. They love you and they think soo yeah I need to marry you. I wanted to marry my cousin when I was younger .... and now my baby cousin wants to marry me .... it's mean nothing
I wish authors would stop using "you promised to marry me" between family members (parent-child, siblings). it's freaking weird.