I am at neighborhood parks quite a bit with my kids. I have noticed that while kids are playing, the adults there with them are often on their phones.

I get why. It is indeed tempting. The kids are running around and occupied. Their activity means that parents might have a few moments to themselves. The park often has benches or places to relax. Why not catch up on some texts or social media activity?
Even without kids around, parks feature plenty of phone use. Walk the dog and read the phone along the way. Try biking and phone use together. Lots of walking with earbuds in or headphones on.
However, parks can be inherently interesting places without phones. Kids are learning and developing skills. There are often hints of nature around, birds to spot, bodies of water to observe. There is plenty of people-watching to be done. If the park is a lively one, perhaps one envisioned by Jane Jacobs where people are using it in multiple ways and it is situated among other interesting uses, there is plenty to see and do.
Additionally, if people are concerned with phone and social media use for kids and adults, could parks be phone free zones or at least spaces where we work to use them less? It is not because it is immediately dangerous in parks – at least, not at the level where I consistently look around and spot drivers around me with their heads tilted down to their phones – but because good parks offer the potential for a respite from other parts of life. If parks, preserves, and green spaces can help restore our minds and bodies, are smartphones part of that equation?
(To be fair, adults are on their phones all over the place. I have just noticed it recently in parks amid my own efforts to use my phone less in this setting.)