Since your goal is to hold thinking on long term results (18 years from now), you should think about storing your coins safely on your own hands. That is, to not rely on any third party online platforms to keep your coins stored. You need a hardware wallet and to control your own keys.
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I'm curious how the OP stored the Bitcoin they bought for the baby, they created a paper wallet? Bought a hardware wallet or any software Bitcoin wallet? It would be more amazing for OP to share how they manage to store the Bitcoin so just in case in the future, other will follow it too.
Just like me, I am planning to do it in the future for my children. Since I saw a lot of people having life insurance to their children while at very young age, so for me, I will do like what OP did.
i just wish that the OP was using Hard Wallet. Because the main problem in the long-term storage for years from a crypto asset is where it is stored. some of the cases that I have seen, many people hold their assets in several wallets, but some hackers are able to access the assets they have. I've even experienced this myself. Anyway, I don't expect him to use CEX to store his assets. I think that way because many lay people think that CEX is a safe and reliable place to hold assets long term.
I also hope that OP's plan is really mature to realize the investment for his baby by thinking about where he will store his Bitcoin assets.
Storing our Bitcoin assets or crypto assets in a secure wallet is very important, what you suggest to the OP is that it is true that it is safer to store Bitcoin purchases for their children in a Hard Wallet for maximum security.
Moreover, this asset will be passed on to the baby, so it must really be maximized in terms of storage security because it takes a long time, of course, to be handed over to the baby when he is old enough.
If OP keep it on a hot wallet, in my opinion, it's not wise, especially if OP only store it on an exchange, it can be bad. I hope OP avoids keeping it on exchange.
But we know Hard Wallet isn't cheap at all, especially since OP is still in a condition where he has to meet the many needs of his baby. So maybe besides OP having to set aside some money to invest for the baby, OP also has to set aside to buy a Hard Wallet.