This is all about preference and what makes you sleep best at night. So, what works for you may not work for me (or others). We use the NY Savers program since they have basic, low-cost Vanguard funds. As of now, we are 100% in stocks.....essentially the fund equivalents of VTI, VUG, VTV, VB, and VO. Our daughter is 11 years old.
Our issue is that we over contributed. We added in $100k in principle (us and grandparents) within the first 4-5 years of her life and it freaking grew to $250k'ish. We are currently using $10k per year for her K-12 private school and trying to end in the $150-$200k range when she is ready for college. So, a bunch of moving parts.
The new Roth IRA benefit is great and we look forward to taking advantage of that when the time comes.
Personally, I wouldn't start moving money into conservative options until at most 5 years out, so 13-ish. Once again, it's a personal preference. If the marketing is ripping in a few years, we will likely move more money over, but if we are in a bear market then, we will likely wait for the proceeding recovery. So some market timing is involved!
Last thing, college tuition may or may not track with CPI, so you need to plan this out for when you move money into fixed income investments.