OMW L., time to confront. Time to set down with the powers that be and tell them that you enjoy having children over but can't do it every day. Pick one day a week that they can come over after school but on school nights they have to be home by 5, or whatever time works best for you. My 9yo goes to school till 4 so after school playdates are out, we only do weekends, which is always an option too, WITH set times!. And this is YOUR house, do not let these children run all over you. You set the rules of your house, write it up on a poster board if you have to and tell them that as long as they are in your house, they have to obey the house rules..if they can't/don't, they go home and can not come back for 2 weeks. If it continues, up it to one month and if it still persists, stop it all together.
Children push the boundaries all the time and they know they can get away with this behavior at your home. Don't let tears or back talk stop you. If I caught them taking things from my house, it would be an automatic suspension of playdates until I talked with the parents/aunt/whomever is in charge.
Some parents just don't invest the time/energy into their children the way it seems you do. So you are perfect for them because they don't have to take care of their own & you get all the work. Chances are they don't know what their children are up to, so it's time you set the rules, enforced the rules & opened up & communicate with their authority figures.
I think sometimes people confuse confrontation with being bossy or in charge. Confrontations often are good in that you don't get ran over by someone & your thoughts and beliefs are revealed. So take a deep breath & just jump in.
Don't be afraid you wont be liked, we all want to be liked, but we're not suppose to be our children's buddies nor their friends buddies, we're their parents. And while we want a great relationship of trust & closeness with our children the truth is we are in charge of them and as such we set the rules. Same with their friends. I have confronted my childrens friends on many occasions from what they are wearing to how they are behaving toward someone or in public. And if they don't like it and chose not to return to my house, it's no skin off my nose. If these kids decide they don't like your rules and don't return...you win! LOL. Your children may not like that their 'friends' don't come over any more, but that's when you tell them about how their behavior was unacceptable & your job is to protect them & that's what you did.
Sorry this is long, but I get so upset when I see parents afraid to confront their children or another child in their care about such things. Not in a mad way upset, but in a sad sort of way. I wish you lots of luck, will pray for you as you confront these children & their parents.