Well that depends on what kind of "kid" they are.If they are new to the game of airsoft,I would not go overboard on the price you pay for the gun,you don't want to spend $300-$500 on a humans then find out you/they do not like the sport.I would recomend a crossman,or d-boy,something like that.Then if they are really starting to get into te game,get yourself/them,a G&g,my favorite type of airsoft gun.Preferably a gr15 or a m4.
Nerf gun
The best airsoft gun you could get for $150 would probably be an Aftermath gun, but you could get a JG M4 or M16 for under $150
The one you can afford
If you are just starting airsoft i would highly recommend an aeg, (airsoft electric gun) but if you are pro and experienced, a trained camouflaged sniper is the best, an invisible predator. However it does depend on the user of the gun. If you are asking performance wise then g&g makes the best airsoft guns on the market. (currently in the US)
I say twin ozez are the best cheap airsoft guns
dont get that gun walmart has the best beginer guns
it depends on what cheap is to you
Definitely Nerf.
www.shortyusa.com
The best airsoft spring pistol for young kids are guns that aren't extremely powerful, I'd suggest this gun which is at 235 fps, others will feel it but it won't hurt too bad. This gun may be purchased from Hobby Tron at hobbytron.com/AirsoftGuns.html, this is the title of the gun 'Sprind Tactical TMP Machine Pistol FPS-235 Electril Sight, Silencer, Flashlight Airsoft Gun' also, this guns runs around $15.95.
The best way - go to near airsoft workshop, give them money and tell what do you want. If you decide to modify airsoft gun by yourself - be warned, this may ruin your gun and even be dangerous to your health.
"Systema" is by far the best airsoft gun brand out there.