I'm doing the same with my nephew,

I gifted him 0,01 BTC back in summer 2018, and I'm now considering to start accumulating 25-50€ every month for him exactly for that same reason, because in 12 years (and he being 18), he will have without noticing it a net worth of 20-40K€ (assuming BTC in the millions).

Good luck with your plan!