On getting a folder for an 6-7 year old.

I looked in to this and tried a lot of folders out main problem was the brake levers difficult for the smaller hands. Brompton did not work well at all. Adjustable v Brakes worked best with soft blocks and teflon brake cables to reduce friction.
Some adult seat post are too long for a child's bike.
Dahon sweat pea?
Mini onza worked also.
Personally we had the best results with a Dahon curve, but we had to get a telespoic post to avoid the fully inserted post catching on kerbs.

Dunlop or decathlon bikes might also be worth a try. I believe sesame seed crunch stated his flamingo was the best bike for a young child they had tried.