I’m trying to write a story about a lady who has lost her son in the park can someone help me with decriminalised how the mom would feel and the setting and all that. I’m really stuck :(

The mom's heart will most definitely be racing.
She will feel helpless knowing that her son is out there on his own.
She will be desparately trying to rationalize the situation as she tries to find him.
If she's married, she'll be trying to call her spouse, or she might also call other close relatives.
She'll be praying that the worst hasn't happened to him yet, and that he's alright.
She'll be looking throughout the entire park for as long as possible, and perhaps calling out his name as she looks.
She might ask strangers if they've seen her precious kid.

Now for the setting, this also depends on your story.

If the son is a very young kid, around the age of 4 maybe, then most likely his mom took him to the park while the Sun was still out and shining. Of course, depending on how long they were there, the time of day could've changed a bit. If the son was around 6 or 7, chances are he might have after school activities and so she might take him to the park a little later, but they probably won't stay as long.

Since the setting is a little more complex, I might have to help you in comments or something. We can talk more if you still want some help :)