It’s a tricky few years - 18 through 20. The years my own were 21 but many of their friends were a year younger were worrisome for the same reasons we are discussing here, in terms of liability for my 21 year old kid providing alcohol to underage adult friends.
Here’s a link to information about social host liability. Honestly, in the OP’s situation it’s most likely fine to serve champagne to her guests. But it is good to understand the liability a host has when doing so.
https://injury.findlaw.com/accident-injury-law/social-host-liability.html