My dog goes to a doggie daycare/training place twice a week, for a full day (she's there today). She has been going for nearly 3 years, since being 5 months old. She plays with rottweilers, german shepherds, staffies, mastiff, ridgebacks, bull dogs, spaniels....you name it, mongrels, too. They assess the dogs (and owners) first and will not allow any aggression. Puppies are kept separate at first and socialised with well-mannered dogs. The staff are qualified dog behaviourists (or studying for the qualification) and they receive ongoing training. You wouldn't know there were 30 or 40 dogs there most of the time, as the dogs learn the 'no barking' rule. All you hear is the voices of the staff giving instructions and praise.
There are usually at least 4 staff on duty and they structure the day with training/obedience, agility, games, rest, learning the odd trick, manners with food (honestly!) and playing with all the equipment and toys. There's never been an incident of aggression, and the dogs behave like one large happy family. They have birthday parties when it's a dog's birthday - for a reason - to train the dogs to wait for food in the presence of other dogs, and not to react if it is removed when they're eating.
For all their expertise and control over dog behaviour, they would never leave one or more dogs unsupervised with a child of any age, especially if the child had food. My dog has really benefited from going there and is wonderful with children, but I wouldn't leave her unsupervised with my grandchildren, especially when they are eating.