Brompton G line childs seat with handlebars and backrest.

For any future travelers, best G line seat I've found was from BoWood but custom ordered via seller on AliExpress (article 1005008803601754). Comes in multiple seat variations, need to specify which one you want. Bundled with harness, which I did not mount it yet.

You can see on photo S size with seatpost set for 160cm rider. The best part you can keep the fold functionality, to roll the brompton into storage or whatever.

I'm not related to ad anyhow, but really searched a lot for the seat and this seems to be the option which ticks all the boxes for me.