Will you really be able to Hold the bitcoins for your baby till he/she becomes older? 18years is a long time and a lot of things can happen in that time.
If you have enough knowledge and faith in bitcoin, nothing is too difficult to do. 18 years is a very long time, but if you are determined, you can do it, it's like saving for our children's future.
If you want to invest in bitcoins for your child, you need to be invest a huge amount and not crumb by crumb.
Why is it necessary to have large amounts of money in the first place to invest in bitcoin when we can accumulate over time? Accumulation is like saving, every time we have lazy money, we will buy bitcoin, and if we maintain it until 18 years later, it will be a huge amount of money.
Also before you child come of age, you have to ensure that you have done the part of educating him or her well about what bitcoin really is, so that at the end when you hand them ownership of the bitcoins you have been keeping for them, they will actually know the value and not waste it.
Even if you don't have any bitcoin savings for your kids for the future, when they're old enough, you should teach them about bitcoin because that's the future.