When your on chemo drugs, they wipe out your white blood cells, which fight off any infections.
It was explained to me that if a count of 10 is normal for white blood cells, then after a dose of chemo the count goes down to about 2 or even less. Leaving your body very vunerable to any germs or virus's, and because you have very little immune system you could become really ill with something that you would normaly just shake off.
It takes about 3 weeks to get the white blood cells up to normal count, but then you have to have the next dose of chemo.
As a precaution your mother-in-law should keep her distance from anyone who may have anything that she can catch from them.
A cold may not be so bad, but childhood illnesses like chickenpox or mumps for instance could be really serious for someone on chemo.
Your children are at no risk from their nan, but she could easily catch something from them. So I agree with your mum-in-law, avoid kissing children for a while.
Ive never heard about wee being toxic from chemo.
Its a rotten thing to go through, and I send her all my best wishes. Its 12years since I had mine. So good luck. It does work.