Charcoal has been linked to gastric upset and seizures, mainly when large amounts have been used when treating poisoning cases.
How have you been feeding your dogs - one lage daily meal or smaller multiple meal times? Sensitive pets often benefit with smaller meals, and reducing food gulping by placing large rocks in the food dish to slow 'inhaling' food.
Also, whenever changing foods, avoid immediate changes so blending ingredients over a few days can reduce stomach upset.
As always, have fresh water available at all times, especially during warm weather.