If any of the people have deep set eyes, what
@Designer said can be important (speedlight or reflector) to lighten up the deep eye sockets.
Depending on the intensity and angle of the sun, the deep eye sockets can go almost black, without some kind of fill light.
You will/may need an assistant to hold the reflector(s). A reflector is a big sail, so you need a person to handle it, not a stand.
A similar problem occurs with people with photochomatic glasses. They go DARK in the sun.
Ask them to bring a 2nd pair of glasses that is NOT photochromatic.
Plan B. I don't know where you are, but when you do your recon, look for alternate sites at the park (or elsewhere), if the weather goes bad on you; rain, snow, wind, 100F+ heat, etc.
Watch the hair color/background. Dark hair against dark green foliage, and the hair can 'disappear.' Been there, done that

I've seen similar with blond hair against an orange sunset, where the blond hair 'disappears.'
When you do your recon, I would bring a friend with you. Preferably one with similar hair color to your subjects. Then do test shots at the various locations at the park.
As
@tirediron said, discuss the color of the clothing that they will wear.
You want them to stand out from the background, but not POP. So no BRIGHT colors. Too bright and the eye is drawn to the clothes, not the faces.
I tend to like my group in similar color, but there is an argument for mixing different colors. If you have mixed colors, the colors should all work together/match, not clash. Have this discussion with the mom, she probably has the best sense of color matching.
If you have sample pix of various portraits, that can be a guide to discussing what poses they want, and what you want to try.
Then you need to STUDY the pix, so that you can set them up for the pose.
gud luk