Puppies will vary a bit... none quite the same
A few tummies may be sensitive but that can also be a problem for a few adult dogs too. My Gracie had some issues with that but very few Cavaliers have the protein allergies that she has. She is now on a raw dogfood diet, but it is a necessity for her situation. I would look into getting a high quality food, but make sure you transition VERY slowly from what the breeder fed. Some do not react well at all to food changes if they are done too fast. I would not give human food as treats at that age...but later small bits of apple (no peel or seeds), or melon may be a good start and can be added to their food. Small amount of raw canned pumpkin (not the pie filling) is good for constipation AND runny poop...at least it has been for us.
I agree with Brian that it is early for bones. As a caution, Gracie can shred up most hard chewies very fast and some can upset her digestion, so we mostly stick to small deer and elk antlers which last forever and do not upset her. Nylabones have not worked well for her either but I think are fine for most Cavs. Just be careful to remove them once they are starting to get too many sharp edges that can upset their tummies. We filed ours down but Gracie just demolished them toooooo fast.
In the car, we have a booster seat with a very secure harness system. It has very high sides but one small indention for her to prop and look out window. We use the following harness when walking her. http://www.petsmart.com/product/inde...ductId=3046157 It is not perfect but she likes it better than other options.
Hang in there with your search
We have been looking for second Cavalier to add to family for a few months and know the right opportunity will come along. I too had to walk away a few times from cute dogs because the breeder or situation was just not right.