I'm aware that it doesn't work like a bank account. I've been reading about possibilities to earn more crypto simply by holding it. One option is staking, but it sounds like there are other possibilities as well, but I haven't looked that deep into it yet, and it also seems a bit complicated. That's why I'm asking.
Staking for children investment purpose? It seems wrong because you might be entrusting this coins to centralized exchanges and just like Darker45 stated it wrong. It looks as though you are looking for a sort of way to earn more from this savings and if you toil this part you might be ruining all savings.
I was thinking something like 50% BTC, 25% ETH and 25% others (is there some sort of crypto index of the 20-50 most popular or valuable coins that you can invest in?) Does that sound sensible?
Trading Altcoins is way too risky than bitcoin not to talk of having them as saving. No matter where they stand at on the altcoin log now some of them wouldnt last 5 more years. Terra Luna looked promising than some we had now but you saw what happened imagine some sort of that happening to any of your picked Altcoin. Stick to bitcoin or I would rather say do your research but dont gamble on the children savings if truly thats your aim of saving this funds