The convertible bag is approximately 55 cm long from top to bottom. I have a feeling this would be way too big for her.
However, as mentioned, the Appenzell is even suggested for kids. It's approximately 45cm in length and weighs half as much as the convertible carry on.
But, there are also numerous wheeled bags sized for kids that will meet the carry on size limits of most airlines. Is she going to be able to handle 15 lbs on her back? If you don't have a backpack, borrow one, fill it approximately with what she is going to be taking, and let her take a walk to see how she handles it.
I did a search on Amazon Canada for "rolling bags for kids" and got this:
https://www.amazon.ca/s?k=rolling+bags+for+kids&crid=296O9HOY36ISI&sprefix=rolling+bags+for+kid%2Caps%2C190&ref=nb_sb_noss_2