Good reason half-hour preview shows for Saturday-morning network schedules aren't done anymore.
The only reason ABC, CBS and NBC still run children's programming is to satisfy the FCC "E/I" rules.
And each network only airs three hours of children's programs a week. Most of these shows are recycled programs from the Disney Channel (ABC), Nick Jr. (CBS) and Discovery Channel (NBC).
Had the "E/I" rules not existed, I suspect ABC, CBS and NBC would all have given up children's programming long ago. They probably would only be programming two hours on Saturday mornings, between 8 and 10 A.M. ET, with weekend editions of their early-morning news shows. The rest of Saturday mornings would have been given back to the local affiliates, where in large markets, ABC/CBS/NBC affiliates would likely have preceded and followed the network morning shows with local news (7-8 A.M. and 10 A.M.-12 Noon ET).