...just as dukkllr said, a Red Ryder Daisy BB gun can be an excellent trainer. I had one as a youth.
But a word of caution. Never let a youth out of your sight alone with a firearm, regardless of how innocent it may seem. Practice gun safety at all times with an adult standing by.
Now, concerning left-handed operation with a lever or pump action, there are no special concerns except, of course, the need to determine which eye is dominent for aiming. As you know, shouldering the gun to the left shoulder, sight down the barrel with the left eye. Sorry, but there are times when a novice will use the opposite eye.
Finally, check out the hunter safety courses available in your area if you aren't familiar with the basic safety rules handling firearms. BOTH your youthful gun handler and you can use the great things learned in this course.
Jim