I have recently stated elsewhere that earth beginnings should not be taught in the public school system that these things should wait until post secondary education when reasoning and thinking skills have been honed a little.
So in sunday school why make it any different, sunday school is set in place to help parents in the spiritual development of a child, to help to give them an understanding of the christian faith. Taught in terms they can understand.
Still no need for kids that have not fully learned how to use their God given reasoning abilities to start in a subject as complex as the beginning of the earth. A subject so complex that even those adults who study it do not fully comprehend.
In addition to the above the school is no way allowed anything to with God so sunday school has become increasingly necessary to give children a place to learn about the spiritual, as prayer is not even allowed in schools. Teachers can't even supervise a prayer meeting on school grounds, pretty paranoid of a God who doesn't exist.
So no why do something like teach evolution in sunday school that makes no sense.