The H-Manga i'm trying to find, has two young children, a girl and a boy (around elementary school?), coming across a couple having sex in public on a park bench, it then skips forward several years and the children (now high/middle school students) have sex outside in a playground/park

If it helps, it was an exhibitionism manga, that MAY have been part of a tank (not 100% sure).
And if i remember correctly they had sex on top of a (circular?) platform (with railings?) in the playground/park, and after they had sex they lied down on the platform for a while.

Sorry if it's too vague, i read it a while ago and can't remember a lot of details

If you know anything similar please tell me. Any help would be appreciated!

Thanks.