Do any other dad's invest money for their kids future?

My lad is 3 and I'd like to invest the money given to him by friends and family (around £3k so far) in index funds to compound over the next 15 years.

I invest myself (primarily S&P500) as a secondary retirement fund by in having trouble convincing the wife to let me do this with his money as she sees investing as gambling and too risky.

If you do this for your kids, how did you explain it to the other half and what's your methodology for investing?

Edit: I should mention in UK based but I understand that there's usually similar schemes like stock ISAs etc