No, they don’t. There are no clinical trials (only weak, observational evidence) that suggest we need fiber in our diet.  That doesn’t mean your kids can’t have fiber from the least-toxic plants (e.g., avocados or acacia fiber), but I would definitely not offer them whole grains because whole grains are loaded with inflammatory toxins and antinutrients that can hamper the absorption of other nutrients and wreak havoc on the gut microbiome.