Depends on the kid for sure..
I've always been of the mind that bb and pellet rifles are a step up from firearms for serious practice after the four rules are ingrained.
When a young shooter is being trained, it is necessary to have 110% supervision regardless of if they have a bb gun or a .22. The implication has been thrown out there in this thread about supervision being necessary when they get a .22, leaving the unstated but implied "not necessary" or "not as necessary" supervision for a bb/pellet gun.
Let's not go there. Consider this, the kid is being trained to be safe and to learn the four rules. I think this is better accpmplished with a real firearm than with a bb gun. Almost no one sees a bb gun as a real firearm, so supervision is likely to be lax. Wrong decision in that what is trying to be done at this stage in a kids shooting life is the ingraining of safety rather than marksmanship. Violations of safety protocol may be dismissed or overlooked because (it's only a bb gun), which would be missing the point of this stage of training.
Would you go answer the phone if the kid had a .22 rifle? Of course not. Then don't leave them alone with a bb gun. When the parent sees the bb gun as less than a firearm, the parent may unwittingly plant bad seeds in the kids training.
Would you be as hard on the kid if he swept his sibling with the muzzle of a bb gun than if he did it with a .22? You'd better be, or you're planting bad seeds in him. Not his fault if training protocol is lax. IMO, you should reconsider the Chipmunk rifle for this reason. The extra urgency will be there in your mind to maintain a higher level of safety and supervision, which the kid will pick up on and be safer faster.
I had my kids shooting .22 single shots off the bench at age 3 and they were shooting 223's from the bench at around age 5 or 6. They didn't get bb guns until later. They had problems playing with squirtguns because of the training they received. It was a little funny and quite satisfying to see the kids hesitate or being muzzle conscious with squirtguns due to the seeds of safety being planted in the formative years of <6. The safety mindset carried over and they were keeping loaded guns in their rooms at an age that some would call irresponsible parenting. Empty chambers were the rule there.
No tradgedy's ever happened from me taking this approach but once again, it depends on the kid AND depends on the parent. If you start the kid on a bb gun, will you have to raise the level of expected safety once you get them a real .22 rifle? You shouldn't have to! That's why I think a .22 rifle makes more sense from the get go.
YMMV.
