We cover this question extensively in our BLOG.
There are 3 simple answers.
Safety
Children should learn Backstroke (or a version of) as a safety stroke because this will be possible before they can perform Freestyle over distance.
Confidence
Children can swim further at a younger age with Backstroke. The tangible distance gives them confidence and the motivation to continue learning.
Speed of learning
When a child learns Backstroke first, they build the foundational strength and core stability which makes Freestyle (in particular correct side breathing) easier to learn. Basically, they will swim 20m Freestyle at a younger age.