The series decision is different from the book decision
A single book decision is contained. A series decision keeps going. Your child may start with a book that feels right for their age, then move into later books where the characters are older, the stakes are higher, and the content has changed shape.
That is why parents need to look beyond Book 1. The question is whether the full path fits your reader right now. A good series guide helps you see the turning points before your child is already emotionally invested.