Wondering if Bitcoin can reach a Million dollar for this to have an best increase because in that time after 18 years the education expenses will surely increase so much high also so meaning if that 100dollar now amounting to 0.0025 so how much would that be in that year?
I'm afraid that the amount you invested will be enough when the educational funds needed that year .
I am quite happy with what the OP is doing, I think his good intentions to provide a good education to his nephew are certainly very worthy of our appreciation, but as you said the value of the bitcoins he is currently saving is quite small, maybe even after 18 years The OP nephew only gets a small profit from the bitcoins the OP keeps so far.
Let's say now OP has bitcoins worth $100 or 0.002, and after 18 years the price of bitcoin has increased to $1 million, so OP only has savings money of 0.002*$1,000,000 = $2000 and of course it's not that big of money in 2040.
If only we are sure that Bitcoin still exists 18, 20, or 30 years from and the price is growing, many people had done this but because we won't that is really risky.
We can't assume anything in the future nor can do calculations because the chances were not going that way.