Is buying Bitcoin or cryptocurrency good for Kids?
Yes because bitcoin is for everyone, it is made for both young and old ones, a child can have it as an inheritance from his parents when they Invested on it and hold to a certain time they feels the child could take hold of the currency.
How do you teach a kid to secure Bitcoin incase you are not there?
Use any means to make sure they understand it, this also has to go with their own decisions as well on wether to accept bitcoin adoption or not after you might have helped in getting them exposed to it.
Should authorities or agencies place age restriction on owning Bitcoin?
No, and even if they wish, they can't because it's a decentralized digital currency that uses p2p network on blockchain.