I've put £70 a month away for each child since they were born. It's then their money for university/a car/house deposit/etc. They know it's there and will be what they get, with us not providing anything else.
Son no.1 got his at 18 and, thanks to a family member part-funding him, hasn't touched it whilst at university. He lives very frugally so has a decent sum.
Son no.2 gets his next year, with a small bit extra from the Child Trust Fund. He's had bikes though and we've funded his racing.