Ok, they are gone.
I have been running around like a mad woman until 2 am last night, up at seven today, got them out for a long walk for the whole day which keep all the children busy and away of the house. Wish sometimes Mumsnet had something to say when a poster is logged in... I was not ignoring your posts, I was dealing with the situation.
I won't bill them for the broken toys. TBH I think that's even more rude than breaking the toys. However, I'm going to be reminding DS about this often, I asked him to hide the toys away before they arrived, but he wanted to show them to the children, I told him there was a 3 year old coming over and he insisted not to take anything out. So as I said in my first post, is dealing with a jetlagged-overtired-recently spoiled-rotten child who was challenge even before they arrived and the toys were broken.
One of the other children has ADHD but despite the level of activity he is a very obedient child, his brother is way younger, he is a sweetie but he needs more supervision, his mother... well, I can see a very very tired woman who has also raised her threshold in behaviour expectations as otherwise she would be driven up the wall (My sister has a child with autism and my exhusband has aspergers, I have been parenting on my own for 3 years now, so I know where that tired look/lack of response comes from). So, it is not that the children were bad it is just that the mixture of the three was nothing but explosive.
BTW, DS is not a doormat, actually, much of the stress was about keeping his good manners in place, he is very resilient, patient, and normally a very caring child, but if somebody push his buttons for long enough, he can lash off, which I obviously, as any other parent, find unacceptable.
So well, to update on the day, I came down a bit after my last post this morning, found the kitchen clean, the living room tidy, all the children and adults fed and a cup of coffee was swiftly prepared for me, I explained that I had an important call to make so I needed to plan the day (my mother is going into hospital this evening/morning but with the difference in time use, I couldn't call her before leaving the house). They told me they were leaving before the evening, so we packed all of the children (and luggage) in the cars and went for a long walk. DS was so tired, cold and fed up he had his first "tantrum" in years. The other children were also tired and cold so they ended up taking us all to a restaurant for a late Sunday lunch, after that we took them to the station.
And yes, they are my friends despite the mill we have gone through, that's why I put up with this, because we have a "history" together, they were fantastic friends and TBH, being all of us expats and thousands of miles away from our respective countries, there was a time when I considered them part of the family, so it is nice to have them back in the country.
Now, for next time, I will ensure DS and I are rested and every single breakable toy put away before they arrive. I guess DS will listen this time.