Try looking at Gamo air pistols. I started off with one before going on to a Fas. They are small and lightweight for little hands.
You can get gas canister pistols but I always thought they were a risk as if you forget to charge the canister and your gun doesn't fire then that is tough luck as you should have checked and you loose the shot (I have seen this happen quite a lot of times). A normal pistol which you 'pump' yourself when you load is a much safer option. I did have a malfunction once when the seals broke but because it was a malfunction I was allowed the shot again.
One thing to remember is not the wear any baggy clothing, scarfs etc that could get shut in the gun when you load for you child. I always found it so embarrassing when my mum would shut her jumper in it by accident as the only way to get her clothes out was to hold everybody up and get the steward to fire it.