A lot depends on the breed of mastiff that you're thinking about and the age of your kids. No matter how sweet the dog may be they are huge and don't know their own strength most of the time, especially puppies.
As for health problems you need to find a responsible breeder that has tested both parents for such problems. The adult offspring of the parents means a lot too. You'll pay a considerable amount of money for this but it will save you in vet bills later.
Then you need to make sure that you're feeding the proper food. Giant breeds never get puppy food as it makes them grow too fast and their bones and joints can't take the rapid gain in weight and bone mass. There are large breed puppy foods but you would be much better off with a food like Canidae or Innova, made entirely of human grade meats and these kibbles have 50% more meat in them than other kibbles. The Canidae is an all life stages formula, so he'll eat it his entire life. It's a bit more expensive than the other "premium" brands but it's known to help heal and prevent skin problems (including allergies) and hundreds of other things.. He'll also eat less of it because other foods have grains and fillers that just pass right through his system. An adult 100lb dog will eat 4 cups a day of the Canidae, or up to 7 cups of pedigree.
www.canidae.com
http://www.petperfectexpress.com/feedingcomparison.html
Then you need to do some research on vaccinations. A lot of the current research is saying that annual vaccination is causing lots of health problems in dogs, such as skin problems, chronic ear infections, tear duct problems, and the foot licking habit some dogs develop. Depending on what serum your vet uses you will likely only have to vaccinate every 3 years (after all the original puppy shots and boosters) and for many things that vets vaccinate for once in your dog's lifetime is enough..
I hope that gives you a starting point, anyway.. good luck..