What type of pellets are they being given? (Since you're in the UK, you may want to list the ingredients as well.) If you are feeding an alfalfa based pellet, you will want to give primarily timothy hay. If you are feeding a timothy based pellet, you will want to give primarily alfalfa hay. The change in the size of their droppings may be attributed to the fact that they were switched suddenly to a new food instead of doing it gradually.
As for treats...
Raisins, bite sized Shredded Wheat squares, raw almonds (no more than two a month), dried cranberries, plain Cheerios, dried apples, a half of a fresh or frozen grape, very small pieces of fresh or dehydrated banana, uncooked pasta, horse cookies, dried papaya or dried pineapple (said to help prevent hairballs), rose hips (a source of Vitamin C), rolled oats... The list goes on! Remember, everything in moderation. Too many treats are bad for chinchillas and can greatly affect their health and well-being. No more than one treat should be given each day.
