@Marzipan Whoa mate... you went from how to I start to what is pro level Im gonna do everything in this thread.. You gotta watch how threads develop here as many will drag you off topic and around the block and have your head spinning.
Lets get back to the basics about the quad for a 7 year old.
To answer some of the questions.. YES you can change from 2 to 3 to 4 blade props on anything. BUT its not just a matter of grabbin em off the shelf and slappin em on. Motors and power trains in our gear have various ratings for RPM, voltage and current draws. No matter what size you fly get that combo wrong and it doesn't fly or it lights itself on fire mid air.
Toy grade aircraft are just that. They are toys they are not designed nor can handle many modifications. Use them as a learning tool to get the basics of flying down. Get a little muscle memory into the hands and get the mind trained how to react to the motion of them to lessen the chances of "Panic" crashes.
As for the batteries I am glad you are active in your grandsons life and excited to fly but DO NOT let the boy charge the batteries. Let him watch and learn maybe let him connect and turn on the chargers IN YOUR presence but DO NOT LET HIM do it alone. Lipo batteries no matter what size are no joke. You are not talking leaking chemicals like old alkaline batteries you are talking fires that can potentially burn your house down.
NEVER charge any batteries where you are not at the very least in the same room with them the ENTIRE time. When they go bad they do so quickly Its not worth the risk of losing everything over a toy. Fire extinguishers wont always put the lipo fire out so don't rely on one to save the day. Keep a bag of play sand at the ready as that is the fastest way to smother a lipo fire.
To be honest the biggest advice we can pass on to new to rc flying people is to start by using simulators. That is the best way but we also know that its hard to not jump in nd throw something in the air when its so easy with all the new tech.
You have experience on your side (AKA knowledge gained from age) Use that along with common sense as you will get many peoples opinions about slow and steady vs dive right in. Many thoughts about flying exist here from the we need rules and regs and you have to follow these steps exactly before you can fly down to just put something in the air and do what you want screw the rules and common sense.
