Am I the only one wondering why the hell a five-year-old is being suggested an engagement?...

As a political agreement between kingdoms. She wouldn’t get married until older, but it would be used as an alliance between two kingdoms to try and prevent war. It was common practice for royalty or aristocracy in the past. But in this her parents are apposed to this and want her to have a normal childhood, but they do not necessarily want to go to war so they are agreeing to host the child, but I don’t think they accept the engagement.