Of course, if he can keep the investment for so many years, he can earn 5x, 10x instead of double. But I think he should gradually increase the money invested there. Because he is investing in his child's future and I think his child will thank him when he learns and understands about Bitcoin.For that if he gradually increases his modest amount here then surely he can take a huge amount of profit from here which will have a good impact on his child's future.
We are just speculating at this time because we are talking about say more then 18 years when Child will become adult and we don't know what bitcoin status will be at that time but we are being optimistic about its growth.But yes it will surely be too high in comparison to current prices so as a parent they can put more regular investments for his future to have considerable amount with him.So overall good decisions but after considering all the factors in long term holding like keeping it securely with seed phrases safe and in hardware wallets not on any exchange or paper wallet.