I presume that
as these offensive people are the grandparents of you child(ren), you may not be able to escape all contact with them.

However, if you ever find youself going to their house to drop the grandkids off, take a few pocketfulls of cherry tomatoes and babybel cheeses.

These are almost completely odourless and too small to be easliy identified when in your pockets and are the perfect size for hiding around the house* (think, behind the wall-clock, on top of the curtain pelmet, in the uplighter lamps etc).

After a few weeks their house will begin to reek to high heaven and they will have no idea why.

If you have the chance, unscrew a fresh air vent on the wall and hide a kipper behind it. This has the same effect but is much harder to carry out due to odour/DIY problems.

For best results, pierce the tomatoes just before hiding and unwrap the babybel.
(Colonel Santiago Introduced surprised kitty to the world., Mon 12 Jan 2009, 13:24, Reply)