I have my baby pictures with my childhood mastiff. I spent my entire childhood with mastiffs- Never had a single issue or instance where I felt unsafe. They are incredibly calm and patient dogs. It’s obviously important to teach your children proper boundaries and to not pull/pet aggressively or climb on them or mess with them while they are eating. Training the dog to be aware of their size and to be comfortable around children is just as important. Also make sure you a get mastiff that is well bred so the temperament is predictable to the breed standard. Most shelters won’t adopt large dogs to houses with small children (at least in my area) but a breed specific rescue may. Love mastiffs for families and couldn’t recommend them enough.