Big Mike and Ayria's comments "sort of" mirror my own feeling about these. I do think these look "over-flashed" because the on-camera flash is the main source of light for the people, and the background looks dark in some frames, and with the on-camera flash being the main, majority source of illumination, the balance of foreground light (flash) to background light (natural light) seems to me to be "over-flashed".
I dunno...I think you need to consider using an off-camera flash that has more power, and a light modifier, to create 1) a bigger source of light and 2) a flash source that is closer in power to the daylight exposure and 3) light that comes from another direction besides right over the lens.
Like Big Mike touched upon, on-axis (meaning right on the lens-to-subject axis) light looks good as fill light, but when on-axis lighting is the MAIN SOURCE of light for the people, it tends to look flat and shadowless, and unfortunately, a bare flash can also look a bit harsh and point-source-like. If the flash were more powerful AND in a bigger form, like say a 30x30 inch softbox, the character of the light would look better. An umbrella or softbox would have made the flash look much more-professional, softer, and just "better".
I'm sure you have seen other people shooting flash in this park, but the way the majority does things is not always the best way. A reflector, like a 72x72 inch panel, would have really been a nice way to go. Or, just allowing the backgrounds to blow out would have been an alternative way to go no-flash, and get an entirely different look (although with their dark clothes, the blown-backdrop look would NOT be my first choice..if they'd been dressed in pastels, it'd be different and more viable to blow the backgrounds out and go for a light,airy summertime look).
At the very least--an 18x18 inch mini-softbox held at arm's length on an off-camera TTL Cord would have improved these photos. Ideally, a 300 watt-second monolight and 24x24 to 36x36 inch softbox or 40 inch umbrella about 6 feet off to the side of the camera would have been my preference. Just flood the posing area with softish, bright light, and shoot.