You have made the presumption that the reason for the diarrhoea is the food that they have eaten at a boarding cattery. That's a fair assumption but the first step is to identify and remove the underlying cause so you've got to really be sure about the cause.
If it is of a short duration, it can be treated at home provided there is not excessive fluid loss. If diarrhoea persists for more than 24 hours it is potentially serious and you should consult a veterinarian without delay.
Veterinarians (the ones I listen to in their excellent manual 😎) say that you should withhold all food for 24 hours and periodically give them very small amounts of water. The cat will recover and then you introduce food gradually feeding three or four small meals a day.
You can begin with a diet high in meat protein or even strained meat baby food. Hill's Prescription Diet Feline i/d is a good example of food for a cat that has suffered from food intolerance. However, it may be hard to find this product. Perhaps the vet stocks it. It is a wet cat food product.
You should avoid high carbohydrate foods and dry cat foods. Foods high in carbohydrates (dry foods are high in carbs) can prolong diarrhoea because cats have a low tolerance for carbohydrates. You then gradually return to the usual food when the cat has recovered.
With veterinary advice only you may want to prescribe loperamide (Imodium) but if a cat has caught an infection which causes the diarrhoea this suggestion may be contraindicated because it can keep the infectious organism in the bowel longer because it may slow the body's efforts to expand it. Also, this drug is apparently controversial and it may cause excitement in some cats.
Hope this helps a bit and good luck.