While most Mastiffs are okay with kids, Boxers tend to be quite protective. Either breed, however, is big enough to accidentally knock down small children just by leaning into them or jumping against them, so accidents will happen between small children and large breed dogs, no matter how gentle the dog is. Also, larger dogs can be a problem if children aren't taught the "dos and don'ts of interacting with dogs (especially large ones). I've met very nice dogs that have issues with food and toy protection (or possessiveness). Young children sometimes think its cute to take food, bone, toy or treat and negotiate with the dog before giving it back or throwing it to watch the dog "fetch." Many dogs will bite when they become irritated enough, and sadly enough, the children are unknowingly asking for it. Then the dog has to pay the price in the end. I would not let young children (under 12) be alone with such a dog until you have trained dog and children the rules and witnessed that they are applying them over a course of 3 weeks to a couple of months, depending on how quickly everyone is picking it up, but no fewer than 3 weeks as unexpected things have a way of coming up that you can't predict nor plan for.