size

Also consider the size of the child.
Many people make the mistake of associating a LOW power airgun with a youth/junior airgun.
However, much more important than power is FIT.
Does the airgun FIT the child.

Think of it like you sitting in a chair made for a 7'6" basketball player.
Your feet won't reach the floor and the bottom of your back won't reach the back of the chair making you slouch.

Figures, since I'm looking for a post with a photo, I can't find it.
So imagine this picture of a child holding the air rifle.
- But the LoP is so long that the child cannot put the butt to his shoulder.
- He has to put the stock under his arm to be able to reach the trigger.
That is an extreme example of a rifle being too big for the child.

The problem is, most mfg do not list the Length of Pull (LoP) as part of the specs of their rifle. So it is difficult to buy a childs air rifle over the internet.
What is the LoP?
On the person, it is a measurement of the distance from the inside of the elbow to the trigger finger, and is measured as follows
- bend the trigger finger like it would be resting on a trigger
- bend the elbow 90 degrees
- measure the distance from the inside of the elbow to the pad on the 1st segment of the trigger finger, that is the measured LoP.
And on the rifle, it is the distance from the deepest part of the curve of the butt pad to the trigger where the finger is placed. Actually it is to the trigger at the point it releases, but that is not easy or safe to do, so the position of the trigger at rest is "good enough."

NOTE: This is only a start to fitting, and is NOT a final fitting number. There are other factors that affect fitting as well; thickness of clothes, width of shoulder, are they fat or skinny, position that they shoot in (prone has a longer LoP than standing), etc, etc. A person can accomodate a fairly broad range of LoP, so don't get too concerned about a perfect fit, just get "close enough." And remember the child will grow, so better to err slightly longer than shorter.

The good thing about a WOOD stock is, a gunsmith can shorten the stock to fit the child. Before cutting the stock, have the smith drill the stock from the back for a few dowels. That way when your child grows, you can reattach the cut portion of the stock using dowels. This is much harder/expensive to do with a plastic stock, and VERY hard to do with a skeleton plastic stock.