Still here .... things I have seen are.
One Lego castle destroyed - not the worst thing in the world, can be remade
Toy box emptied all over floor breaking damaging contents - were the toys old or fragile as toys do break over time. If they were new, then I'd expect the Visiting Parents to replace them
Coffee table books all over floor and being stood on - Tell the children and the parents "We don't stand or walk on books in this house. Please pick them up, put them back on the coffee table and carry on. If you don't, you'll be on a time out, isn't that right *John
Water being poured into food - not sure what the issue is here but I would have removed the water at the time from them so they couldn't do this, even if it was their drink at mealtime. They could have a drink afterwards.
Sticky hands all over walls (ie getting down from table and putting hands straight on to walls - Napkins and a word to Mum or Dad of Visiting Parents "Please wipe *Jane's hands before they leave the table"
Eating with hands (not finger food, think pasta) - I'd have to have a word with Visiting Mum or Visiting Dad about this "We use cutlery to eat at our dinner table" feck that if they think a comment like that would be snobby, it's just basic table manners.
Standing on chairs at meal times - Talk to the kids before dinner is served "We are going to sit down for our meal soon. You'll sit down too, wont you and show us how good girls/boys sit at the table for their dinner?" if they get up, remind them "Please sit down at the table *Jane, we're having our dinner"
Running jumping on sofas - Again, you need to intervene "These things are precious to us so please don't run on the furniture when you're a visitor in our house"
Cushions throws chucked off sofa all over floor - this is probably a normal thing to do for them in their house but if you don't like it happening in yours, you must intervene and tell them no.
Teeth being brushed from a cup in the middle of the living room (parent doing it) - was the parent brushing the child's teeth or their own? Just say "We brush our teeth in the bathroom so I'd appreciate it if you could do the same".
You need to be more assertive in your own home. Your home, your rules and they need to be followed or the guests will be asked to leave and cut their stay short with you (which will probably have the effect of ending the friendship but they wont stay with you again, so result!)
*names changed (as I don't know their names or yours) 