In my view, English Mastiff are by and large inactive and prefer to be indoors. They are excellent watch dogs and get along well with kids. You do need to keep puppy and kids under supervision. Kids needs to be taught to respect the puppy, making sure they don't disturb the puppy when he's sleeping, making sure, during play, kids don't pull the tail or step on the paws of the pup. The bottom line is always keep the dog and kids under supervision.
Having said above, you need to know some important things about Bull Mastiffs:
1) Their pups have good appetite and eat good amounts.
2) Mastiffs drool a lot. Can you and your family tolerate that? Keeping a towel and cleaning their mouth regularly is the only rescue to this.
3) Mastiffs have very low life expectancy, they usually don't live more than 8 years.
4) These dogs tend to have joint problems and are usually unhealthy, meaning you may have to spend a lot of money on veterinary treatments.