They need to say no and stick to it. If they are unwilling to do that, there is nothing YOU can do for them. May be give them pep talks and tell them how it can be difficult at first, but the more consistent they are, the easier it will get as time goes on. But they have the choice - do the easy thing and have bratty kids, or do a momentary unpleasant thing and have well-behaved kids. Good luck.

OP here, sorry I forgot to mention that I did give them advice, but they are still having trouble saying no. The kids are not used to their parents putting their foot down.

"Say what you mean. Mean what you say."

That would be my advice to the parents. If you aren't willing to follow through, don't say it. If you are willing, ignore the tantrum and stick to what you said and they will learn that you mean what you say. Will take a few days of 'testing' but should settle if they are really consistent. If they aren't willing to be consistent, don't even try to correct or you just make the negative behavior and tantrums worse for next time.

I don't get it. They're asking you for your advice, seemingly because you are successful w/ the kids and the parents need help? And you're at a complete loss?

Help the parents. Why are you "at a complete loss"?

I'm not sure exactly what your question is.

Like the title said, my MB and DB are asking me for behavior management advice for their
3 kids (ages 3, 5 and 7). I am firm, but very fair and the kids and I have a great time when I am with them. I have to handle some temper tantrums, but I do so quickly and the kids calm down. I am teaching the kids manners and that screaming for something doesn't get you what you want.

When MB and DB get home, its complete chaos. I am a live-out, so I get to go home. I know that their parents are tired after a long day, but giving in to the kids as soon as they start screaming and demanding something is teaching the kids that their parents are push overs. Both MB and DB told me that they don't like saying "no" because they don't want to deal with the tantrums that follow.

I'm at a complete loss. The kids are good for me, but their parents are making things worse by not saying no. I will help where I can, but I don't want to over step my boundries.