If they didn't sleep, they are too cold. 16 sq. ft for 12 chicks may be too large an area. Make it smaller for now and expand it as they grow and feather out. Cut down on the total area and monitor the height of your heat lamp....lower if they are too cold, higher if they are avoiding the area directly under the lamp. There should be
enough space in the brooder where they can escape from the warm areas but not so much that the lamp cannot heat the brooder. Adding a top to your brooder will also help conserve heat at night but make sure to leave one portion uncovered to allow air to flow/escape.
You'll know when you need to change out bedding, just by visually seeing the amount of fecal material, if the chicks are getting/staying dirty, the smell, etc. Most oldsters don't change bedding, only add more dry material.
I wouldn't be adding greenery from the yard...chickens graze by snipping off tips of greens and they can only do this if the plant is anchored to the ground. When you just throw in clippings, you run the risk of having impacted crops, and gizzards that have no grit yet for grinding the tough cellulose strands found in greens.
I'm wondering if you could get help and instruction from your source of chicks? It's hard to learn chicks on the fly from the internet if you have no practical
experience with poultry.
Can't really give electrolytes too often or too long....I'm still adding ACV to the water of my flock and they are 3-5 yr old birds.