Don't I have the right to prevent someone else's dog from climbing on/barking at/jumping on me or my child?

Dog isn't getting enough exercise and training. As an experience dog sitter you definitely should know how to train a dog. You need to take the dog out for a good long walk and you need to train the dog to sit down on command.

A dog which is sitting down isn't jumping up on you. Boxers are well know to be high energy particularly when young, so dog needs more exercise. Mental exercise can also be pretty efficient in tiring out a dog.

Read up on mouthing and play-biting, there are plenty of internet resources telling you various options. Not every method works with every dog. Mine reacts very well to making a squeaking sound and he will immediately desist, likewise a "no" will generally suffice, though when he was younger there were times when I had to repeat it a lot.

Persistence is important. You probably should teach him to play tug to find an OK outlet for the behavior.

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