Kids dolls are their little “family” so it’s natural she wants a doll carrier for the bike so her doll can go on rides with her.
I love to see kids playing outdoors and particularly a healthy activity like bike riding.
As a kid I spent many hours riding all over the countryside but in my case it was usually a fishing rod strapped on my bike. We didn’t have head protection back then but please make sure your child has the proper protection when riding.