When my son turned 4 I bought him a CRICKETT 22, these are awesome and pretty accurate as well. They have the peep sight style back sight and this seems to me is a lot easier to pick up on when you are first startingout. These are extremely short and work real well on kids starting out. When they graduate to a scope mine also came with mounts.
The RED RYDER is a reat starter gun, but mine would much rather be shooting a REAL GUN any day than a 22. BUt definitely start out with open sights this teaches them how to aim the proper way and it also learns them eye control, mine is left eye dominant and right handed, man did we have a time, finally made a patch to put over his left eye, this worked great and he wiened himself from it, by first shooting with both eyes open then going to the left eye closing FYI STARTED MY 6 YEAR OLD GIRL SHOOTING THIS YEAR GUESS WHAT? YEP LEHT EYE DOMINANT , RIGHT HANDED!!
The balloon idea is great or DAISY HAS a target out that is approx 10 inches wide and gives sound effects when it hits the bullseye and different ones when it hits any other part of the target.
When they master the open sights (if and when you graduate them to a 22) ask them if the want a scope and if they do go for it.
The biggest thing is to use a gun that fits them, and ALWAYS ALWAYS ALWAYS USE HEARING PROTECTION, THE EAR PLUGS ARE OKAY BUT I WOULD DEFINITELY RECOMMEND A SET OF MUFFS SIZED FOR KIDS!!!
AND THEN BURN POWDER BURN, Always have plenty of ammo when you fgo shooting or they will never let you forget it, and besides the looks on their faces break my heart when they have to quit, so when I think it is time to quit and they have shot enough, play a little game and tell them as long a sthey keep hitting whatever they are aiming at, you will stay and shoot, word of caution sometimes this can be a while and this is when they will do some of their best shooting, but that is OKAY.
Also regardless of whether it is a bb gun or rifle, let them help you in the cleaning process of retiring the guns for the day and they love this trust me and it will start a great lifelong habit, ALWAYS DOUBLE CHECK THE GUN MAKING SURE IT IS UNLOADED, AND MAKE THEM DO IT EVEN AFTER YOU HAVE CHECKED IT EXPLAINIG TO THEM UN LESS THEY CHECK A GUN THEMSELVES THAT VERY MINUTE NOT 2 MINUTES AGO, THAT A FIREARM INCLUDING A BB GUN IS TO BE TREATED AS A LOADED FIREARM
:-D