Between STI ETF and Roboadvisors, which method has lower transaction costs and which is better in the long term (I.e. 15 years)?

Roboadvisors have lower costs, but returns wise can't say which one is better. Stashaway puts money in US market, so you'll be exposed to exchange rate risks. STI ETF is locally and more comfortable if you want an investment in tune with your living environment.

Although for children's education I won't risk my money in investments where the capital isn't guaranteed - just my 3 cents' worth of thoughts.

Go straight into investing in a global portfolio instead of just one country (Singapore). You'll probably perform better over a 15 year period.
Paying an advisor will cost more but since you're asking a question like this, it may good to pay for that advice to help you with your investment decisions.