I don't let my kitties play with foil.
About 10 yrs ago one of my sister's cats died from eating foil . He had surgery to remove it but the toxins had build up too much in his system and he died
Someone asked "Where's the proof" that aluminum is toxic or otherwise dangerous to cats. I submit that this is the wrong question.
With regard to anything we give our cats, we should first look for proof that it's SAFE -- not assume that it is until a tragedy proves otherwise.
Also, we have to bear in mind that there's a lot of bad advice out there, much of it time-honored and traditional. To protect our cats, we should always err on the side of caution.
They sell crinkle balls at the pet store that look and sound crunchy like a home-made foil ball, but they are made of something else-maybe mylar? I'm sure those are safe, and cats LOVE them. They are also real cheap.
You have a couple posts here from people who had bad luck with foil-maybe it doesn't happen often, but it is obviously a risk. I don't get in very many car accidents, but we all wear seat belts.
I personally haven't let the furkids play with foil balls due to fear of them biting off a piece. I think covering a pee-spot with foil is different. The kits most likely wouldn't be playing with it or biting it. They just won't want to step on it.