N0tNowBernard ( a brilliant book by the way).
Just be up front with them "Beers, no I meant snacks to eat. We don't expect a babysitter to be drinking while looking after our baby." smillie face.
Or call them up and basically say the same thing to them.
They are most likely to say 'ah no, of course not.' but if they start an argument about their right to drink beer just tell them you will look for another baby sitter.
I rarely drink but of course do drink when I am in charge of my own kids. So I am not super uptight about alcohol. But if someone is looking after your kids for you then you don't want them under the influence of alcohol, beer singular or beers plural.
If they don't get that, maybe get a different babysitter.
Ethylred "This is a relative, who is doing you a favour. You have a choice: buy the beer, or become the family joke and kiss goodbye to any relative ever doing you a favour again."
That's a really sad view of families.
If they can't sit in your house for an evening without a beer they have other problems to deal with.
We set up a babysitting circle from my NCT group. We agreed to swap with each other so other people looking after each others babies/children were all quite familiar with child care. We wanted to help each other out because we wanted to be helped out at other times.