I want to buy 0.1 bitcoin for my kids. Where is the best place to do this?

I have three kids (8 years old, 6 years old, and 5 years old) and am setting up investment accounts for them.

I invest in bitcoin and would like to buy bitcoin for my children. I believe in the idea that 0.1 Bitcoin may be enough in 15-20 years to have some substantial wealth. My idea is to open a brokerage account like WeBull and purchase 0.3 bitcoin (0.1 for each child). I would give them their portion for their 18th birthday. Is there a better way to do this?