You Can't Fool the Dog. Kids Are Not Authority Figures.
Some trainers (or television personalities) will tell you that you can teach your dog that your 6 year old is a "Pack Leader". For the love of kibbles and bits! Your dog is too smart to believe that! It's good to help your child learn to give cues to the dog. Teaching a simple hand signal for sit is a great way for the child to tell the dog what to do before the dog jumps on them. You can teach children to give simple cues and commands to an already trained dog. However, all but a very small group of very mature children can be your dog’s “parent or trainer”.
The reason why it won't work is that most of the time the child spends in the dog's presence is spent being a child. They run, scream, flail arms and legs, jump, wrestle. All of these normal child behaviors tell your dog exactly who your child is: a juvenile litter mate. Dogs are very good at reading body language. They make judgements about almost everything based on the body language of other individuals, both dog and human.
The best way to handle kids and dogs is to realize that your child doesn't have the body awareness to communicate kind authority to the dog. Supervise. Teach children to be gentle with the dog. Teach them how to signal the dog to sit. Teach them to ask for help when a young dog becomes too excited.
When your dog is following your child as they race through the yard, jumping on them and mouthing, realize that the children are inviting them to play. When there's going to be a lot of running around, give the dog a chew bone and quiet place to enjoy it.