It sounds like you want your children to have unstructured time with friends? If so you need a like-minded community. It sounds like this used to exist at your pool but then older children stopped going. If so you’ll need to find another pool or club or church or maybe even move. There are walkable neighborhoods where kids do live the way you grew up.
However, if you do not want to move or find this community, kids will spend unstructured time without friends or will need to have structured friend time or worst of all by your standards, it seems, kids will need structured activities.
A compromise might be to learn what the kids’ friends are doing and sign them up together. For example, if kids friends are busy playing soccer, then sign your child up for the soccer team and arrange it with the other parents to have the team hang at the park after practice. Chances are your kids’ friends also have a similar struggle with not being in a walkable area and have chosen structured activities as a solution, but would welcome coordinated unstructured time