Play with your dog and it will have mental stimulation. Your part as a packleader will be confirmed. Some dogs are jealous of others possessions and want to have them for it selves. The dog with most self-confident will be winning, but sometimes a dog of high rank let a low ranked dog win just to quit the play. It's natural for a dog to guard and defend things they watch as their property, for example high estimated things, sleeping- place. or attention from special people.
Children always have low rank.
They should never be left alone without adult company if they are together with dogs. Tendency to excite themselves comes partly throught inheritance and partly through learning. It's one of the characters which have had the biggest part in dog-breeding. Excitement gives satisfaction which is the reason for dogs to get this behaviour. They are social and for them it's unnatural to be without company or occupation.