Is this the market price for this mix?
Theres no real market price for mutts, they can range from free to several thousand dollars.
What questions do we ask the sellers?
Given the price you could ask why they are so expensive, have the parents been tested and certified as breeding quality(CERT, PennHIP, etc). How old the dam is, you will want to meet the sire and dam. you could also ask them if they'd take the puppy back if you cannot keep it at any point in the dogs life. you didn't say how old they are, you will want to make sure they are current on their vaccinations.(puppies usually get vaccinated at 6, 9 and 12 weeks, rabies shots can be given any time after 3 months of age).
Our kids are 9 and 12...is this a good age for them to take primary responsibility for this puppy?
Depends on the kids. only you can make that decision. just remember if they dont step up it falls on you to care for it.
We have a 3 year old male cat...how will they get along?
Depends on the cat.
I do not believe this family has any licensure for breeding or selling dogs...should this concern us?
Depends on the state, most states dont require any licensing for breeders unless it's a large scale operation, of course PETA and the HSUS has it in the works for that to change.
If you do get one of these pups be sure to socialize it like crazy(out meeting new people and dogs in new places, everyday) cattle dogs tend to be agressive and to avoid that you need to socialize. both of those breeds are high energy amd will require multiple daily walks to burn energy, other wise you're going to have a destructive pet.