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.
Obviously this part to me will be interesting to know too , because the issues here is not just buying Bitcoin and keeping it from now to 18 years to come the security of it is paramount. Also I do believe that the Op is very much aware or has gotten some Bitcoin education to have taken this decision, The crypto space is not just about doing what other people are doing, it a matter of you understanding what you are doing and what Bitcoin is all about for you to take those decisions.
I commend the Op about the long term decision and view about Bitcoin, it really the best way to start with, and with that the Op would have no worries about the bull and bear market along the line, basically wether the price goes up today or down tomorrow until the child is 18 years.
As for me, I do however believe that the OP has thought well in investing in Bitcoin for her New Baby and I know Hodling for the long term is the goal but I would want to sell off during the ATH and buy back when the market hit around bottom again, that way she is able to double or even quadruple the investment along the market circle.