saving bitcoin for kids is a good idea. but we need to write a wallet and tell them later. because we no one knows when it will die. This idea is good, but it must be thought of how to convey to the child. may be by writing a will.
This is a good idea to save Bitcoin for the next generation. About the security, you can let any of your close relative know the seeds to the wallet. But you never know about the future or in other words you can say future is uncertain which again makes you conscious to invest a little, say buy 1 coin for your kid and he will enjoy good fortune if things went smooth.