There are two dictionary definitions of fest:
a special event where people can enjoy a particular activity or thing:
suffix: used in combination with other nouns to create a word for a situation in which there is a lot of something:
They met at a recent economic gabfest (= an occasion when there is a lot of talking) in Washington.
The documentary is a fascinating clipfest (= a film that includes lots of different clips).
There's no connection between the McLaren family's annual Molly Fest and the OP. The McLaren family's Molly Fest is so-called because they hold a get-together in the memory of Molly McLaren. This is dictionary definition number one: basically, a festival.
The father in #0 describes the event they're going to as a 'molly fest' because there's going to be a lot of 'molly' there. This is dictionary definition number two: 'a situation where there is going to be a lot of something'. (Not a festival.) The OP doesn't tell us what that 'situation' is -- it could be a party, a dinner party, a nightclub, a concert. All we are told is that there is going to be a lot of molly there, wherever 'there' is. (You all reliably inform me that this means ecstasy!)
This second use of 'fest' to mean 'a situation with a lot of something' is so common in English that I think all native speakers would know instinctively what was meant in the OP: somewhere with a lot of molly. Even though I had no idea what molly mean, it wouldn't have crossed my mind to look up 'molly fest' or to associate it with a specific event.
Some more examples of 'fest' used in this way:
This week's Game of Thrones was a booze-fest
Game of Thrones series eight review – a nostalgia-fest for long-suffering fans
The Spärrows, Manchester: ‘glorious carb-fest’ – restaurant review
A Christmas Carol review – twee-free torment-fest is a tonic for our times
Conservative conference fringe meeting was no ‘Muslim-bashing fest’
edit: haha, Dragon said all of this earlier!