I do think it's a weirdly regional thing and very much down to who you know, and definitely quite middle class. I grew up in the rural SW, there were lots of drugs around, but coke was rare- I don't think I was ever offered it. I've also lived in the home counties, Bristol and London. In all three places, coke was rife among middle class young professional types- but in Bristol especially, it was rare for people to do other drugs.
FWIW I think if kids are there, one parent should always be sober.
However, I do think there are reasons why doing coke (with alcohol) is worse than "just" being drunk:
-As others have said, the risk to a kid of doing one line or eating a bit is way worse than them having a sip or even a gulp of alcohol.
-The risk of it being cut with something else. It's rare for drugs to actively be cut with something dangerous- after all, dealers don't want to kill their customers. However, there's always a risk of someone stupid in the supply chain, and you don't know where it's been. I know two people who accidentally snorted warfarin with coke, which is obviously very dangerous.
-If something happened to you/one of the kids, you'd go to A and E, even a little bit drunk, without hesitation. If you've got drugs in your system, would you hesitate?
-Coke + drink is more dangerous than either one on its own. When they're processed by the liver together, it produces a byproduct which makes you more at risk of heart attack and stroke. Also, taking any stimulant and depressant at the same time is bad for your heart/blood pressure and can cause issues. You might feel like you're fine- but you're not.
- The come down is really awful. It's not something that it's fair to subject kids to IMO. Who's looking after them the next day?