I don't think that smokers realise how BAD they smell; I was on a train and one of the passengers had gone out at a station to have a fag. I could smell him half way down the carriage when he returned and one of the other passengers made a face so she could smell him as well .
^^ This.
I think most smokers are a bit deluded to be honest.
Look, you can't stop them. If they don't see the importance of doing it for the children and would rather lie to you than actually make the effort to do the right thing, then what can you do?
If they smoked around you your whole childhood and you haven't fallen down dead and are not carting round an oxygen tank because of it they probably think you are being a bit hysterical.
You have a simple choice to make. Either put up with it, it's only a couple of times a week - and do you want a situation where they can never visit, ever? But encourage your children as they grow up to say out loud how unpleasant they find it if the GP's kiss and cuddle them and they stink, or if they light up in the car and it makes them want to vomit, or if they light up around food. Make sure your children do not feel they need to be polite and suffer in silence. Hopefully that will shame them into making changes.
Or, you will just have to make alternative arrangements for childcare, and tell your parents why. And tell them that the children can no longer visit them unless you are with them, so that you can remove them from the house if your parents won't remove themselves.