Buying BTC for my little one

Hey everyone, I need some (not financial XD) advice. I saw an article stating that buying your kid 5 euro’s a week in BTC for the upcoming years is not a lot right now and will (probably) be worth a lot in for example 15 years. Great idea in my opinion. My son is now 3 years old and I was wondering what would be the best way?

Exchanges don’t accept people under 18. There is one in my country bitmymoney, haven’t heard of this before? Maybe anyone did use this before?

I have a ledger but mixing funds is not my preferred method. Would like the 5 euro’s a week, of per month, to be automatic if possible? Any tips? Thanks a lot in advance!