Question for Brits (with children) going home after ETA implementations.

Recently the UK has made this new rule that tourists need an additional document called an ETA for travel.

I am married to a Japanese citizen and have a 1 year old.

While I understand my wife needs an ETA to travel to the UK, I am unsure about my child needing one.

My child is automatically eligible for UK citizenship as I was born in the UK and hold British citizenship, however we initially decided that he doesn't really need a UK passport because a Japanese passport is pretty strong by itself.

If we were to travel back to the UK, would immigration check on his citizenship and demand that he should be using a UK passport to travel back home?

We thought that a Japanese passport and an ETA would also be enough for our child and a certain lack of information regarding children potentially holding dual citizenship.

Thank you for anyone who has any information.

by Tianshui