I love it when a junior handler runs with the big guys. We have loaned out several of our Master dogs for just that purpose.
Here is a friend running our dog, Jive.
They were both 6 years old at the time. This wasn't actually a Junior test but a Junior Handler's event that the club offered, similar to Puppy Stakes. The boy's father was going to let him run their young dog in it, but was concerned that the dog might be too much. I offered Jive so the boy would have a good experience and he did.
We also loaned our first master dog, Dusty to run Juniors with an 8 year old while Daddy was handling their dog in Juniors. They thought it was so cool to both be running in the tests together. They went straight through 3 tests, passing every time.
But, 4th test - was on the boy's birthday and Daddy thought it would be very cool to let son title the family dog on his birhtday. Dog took advantage and dropped final bird of the day and started plucking it. He had never done anything like that before. Judges had to ask Dad to step in and get control. It was a bad experience for the child and he never wanted to run again.
If you have an upper level dog the child can run, or have a friend with a dog that will run with the child, I would highly recommend that you make it a father/daughter event by letting the child be successful while you run your junior dog.