If you want to impress any kid, buy them a Nerf Rayven (compact and easy to modify) and some overpowered batteries. I have one of them sitting on my desk at work to fend off anyone trying to give me more tasks.
For the batteries, I got trustfire (unprotected) ones off DX but remember: you need a specific charger for them and you need to be careful not to put them in other stuff because they output roughly 3x the standard AA voltage and will burn out many electronic gizmos.
There are 4 battery slots on the rayven so to regulate the extra power you get you can buy another cool thing that I didn’t previously know existed, battery blanks that just fill the empty slots you don’t need. I’ve got 3 rechargables and one blank in my rayven because 4 works, but makes it sound like the gun is about to take off or tear itself apart 
