The general advice seems to be that you will need 1000万 at least. (250万 for HS/750万 for Uni).
The Junior Nisa is not going to be much help here, it is being sunsetted in 3 years' time.
The question is, What approach should I take?
*I believe gift tax law determines that 100万 per parent, per year, is the current limit.Option 1. Do nothing differently. Max out iDecos and Nisas. Keep 1 year`s living expenses in cash. Invest the rest in a taxable account.
(Low fee Global stock index 80%, Developed Bond Index 20%, adjust with age).
Option 2. Tsumitate. Invest 50-60万 every year in his account separately. Do not pool the funds.
Option 3. Front-load. Aim to invest 100万+ every year until age 11. Take advantage of compound interest, and max out the Junior Nisa for all remaining years. Even at the cost of other investments.