Did you meet the kids before agreeing? Were they dirty then? There isn't much you can do about that aside from talking with the parents.

As for the behavior. At 5 and 8 yrs they are old enough for solid, firm consequences.

Figure out what will work the most for them. Some kids work best getting positive praise and earning rewards other kids work best with having privileges removed.

Do they have a favorite movie? Lay down the law, let them know EXACTLY what you will and will not accept from them. Call them out on it IMMEDIATELY. If you think a movie will work then arrange to have a movie party Friday after school. Lots of special treats and a movie. If they behave until then they can watch...if they do not then they get to sit at a table in another room and draw or use a set toy while the rest of the kids get the treats and movie in the other room.

Expect they will NOT watch this first movie but expect they will realize you mean business and next week plan another move day (or some other big event). Lay out the rules again, track their progress. Don't make it too far away that they can't succeed but not so close they don't have to work for it.

Over time slowly stretch out the time between rewards. The goal is that they start to try and monitor their own actions to get the reward that it becomes second nature to them while at your house.

If the movie isn't feasible for you have them earn candy or something else. Give them a reason to behave to try and get them on track. In time they will hopefully behave and you can phase out the rewards.

Sounds like they need to feel the pain of suffering the consequences of their actions (missing a favorite movie etc) to realize they do indeed need to smarten up.