I think you just need to stop accepting their behaviour.
I was letting my kids trash their toy room. The other day I got jack of it and now the new rule is it get's tidies twice a day, before their day nap and then just before bed time. I time it so that it's about 20-30mins before sleep time, I help them because they're only (just) 2 and almost 3 1/2. They are old enough now to learn to be responsible for their toys. DS is still a bit young to toatlly understand it all, be he gets that he needs to help and that I won't take no for an answer any more. If I find their toys where they shouldn't be, I take them to the church next door and donate them. I don't care if it's their favourite toy, they should take responsibility for it. And I have done it. If I give a threat, I do it in the time I specify. So if I say by the time I hang the washing out, you need to have tidied away the blocks into the container, or I will take your toys next door and donate them to the kids who don't have toys....I will do it. DD know's that very well and learnt the hard way. DS again is a bit little to get it, but knows he has to pack away or I won't be happy.
For your kids, being a bit older and able to comprehend consequences, I would sit them dawn and give them an honest forthright talking to. Write down what you want to say to them, figure out a loose plan, and then have them add their input. Tell them how you feel too, when they ignore you and disrespect you and their things. Maybe for your 6yrs old, some visuals would help her, like a sort of sticker chart. Then outline the consequences of what will happen if they don't do what is expected. And do it. EVERY time. You need to be consistent. It can be exhausting for you in the beginning until they realise you're serious, but once they do they should start doing what you want them to and what is expected of them.
Good luck, remember that they are in bad habbits at the moment and it will take time to change, but persist. My kids are getting better at knowing to clean their toys up, they don't resist or whine about it so much anymore, they just help, and I don't have to do as much prompting or helping as I did a week or so ago, because thye knwo I won't let them get away with it any more.