Six year old twins or cousins? Where were their parents when this was happening? Are they just popping in to see you, when this happens, or have they travelled a long way to see you?
Lots of people here are quick to judge your daughter or son for "bad parenting" but sometimes it isn't that simple. Maybe they should parent differently, but you can't always tell them that! You can, of course, talk to your son or daughter, but in the meantime, you can invest in some cheap games which won't break your greenhouse - swingball or boules, plastic golf clubs or croquet set, bottles of bubbles/a paddling pool/ a sandpit (doesn't have to be expensive. We used our old baby bath and put sand in it). Keep these things at your house as special to be played there.
Do not give them chocolate or sweets when they arrive and then complain if they go mad on a sugar high (voice of bitter experience there!) Give books, puzzles, crayons instead, and have a plain biscuit to hand - you can get very cheap packets.
For indoors if you can childproof a room and tell the kids that that is where they need to play. Again, check out charity shops for a few special toys to be kept at your house. Have crayons or books and sit with them to do some quiet reading/crayoning - they may not have been taught how to read quietly in the day by their parents. If they have had a long journey to yours, factor in a trip to the park as soon as you can after they arrive. Exhaust them, if you can. If it's too hot/too rainy take them swimming then have a dvd ready or board games for when they get back. Have food prepared before they come so that you can spend your time with them and don't have to be in the kitchen and keep an eye on them. If they are local perhaps arrange to meet up at the park/swimming pool/ for a long walk (adventure) instead of meeting at your house if you can.
It must have been very upsetting for you, but having grandchildren in your home can be a big deal. If rearranging your house is inconvenient set the rules for when they come there, but try out some other ideas to keep them worn out if you can.