I'm no parent, but for me, my parents were always firm in their decisions. No meant no, and with their scary glare I knew that if I went against their wishes, I would be in big big trouble.
When my first son was tested for ADHD (Attention Deficit/Hyperactive Disorder) at age 10, the psychologist who diagnosed him, gave me some tools to would help. (That was before we learned that the other 3 children also had varying degrees of ADHD.)
Some things he instructed me to do.
Turn negatives into positives (reinforcement took 9 months before any positive change in his behavior was observed.)
Structure the waking hours -- in order for him to feel secure, (it had to be in 15 minute increments before he felt comfortable & secure)
Create a schedule of activities, chores, behavioral responses expected, and the rewards &/or consequences
Set boundaries, where/why to set them. Plan how to maintain them (have plan A, B & C.) Infractions?? What consequences to follow.
DON'T SET BOUNDARIES, UNLESS YOU INTEND TO FOLLOW THRU WITH WHATEVER THE CONSEQUENCES WILL BE.
And know that the children will test the boundaries & your resolve to maintain them.
If you have a partner or husband be sure that you both agree with whatever y'all decide to do.
Once you are in agreement, call a family meeting & introduce the boundaries, the consequences -- both positive or not, to the children -- all ages.
The children may be really upset, so be ready for their "reactions," and don't budge or get pulled into their drama.
Remember you are the parent, and why you are setting the boundaries. Only good can come from this, for everyone in the family.
Regardless of what they, say children of all ages need the security of boundaries, & knowing that their parent loves them enough to stick to what is put in place. Besides that, when these children are grown & on their own they will have to abide by boundaries in all aspects of their new life experience.
I am a good listener, if you ever want to talk.
Good Luck ! Let us know how things are going.