529/Roth contributions/taxable account for education

I am late to the party contributing to my kids future educational expenses as they are 11 and 13 (have $0 earmarked currently for either). My question is do I start contributing now to a 529 (could contribute $5k/yr for each)? Do I use my backdoor roth contributions (would have about $100k cost basis to access) if needed? Or do I put that $10k/yr into my brokerage account and liquidate when or if my kids need it for education at the time?