Thank you for the replies everyone! Just a quick reply before I update tp @sleepyscientist, yes, the kids do drink spirts at home, the last time we were there, they got hammered on wine and gin, and their mum texted me the next day, laughing about how they tried to get out of going to school because they were hung over!
The BBQ was on Sunday and we set up a mocktail bar (genius idea @TeeBee!) and had beers, ciders, wine and bucks fizz as well as soft drinks set out for the adults. We do have a cupboard in the kitchen where we keep spirits but seeing as the get-together started at lunchtime, I didn't bother getting them out and thought someone can just ask me if they want something stronger. Half an hour after they arrived, 14 year old comes over to DH and asks if he can have a rum and coke! Dh laughed and said, "No chance!" and carried on his conversation. 14 year old then started whining and trying to persuade DH that it was totally OK, because he drinks at home all the time, has been drunk loads of times and is always fine! Eventually, DH convinced him that he wasn't budging and 14 year old wandered off and had an epic sulk all afternoon. His parents just stood there with their fucking beers and didn't utter a word!
There was a bit of a stunned silence from the other adults as they watched but once the kid had wandered off, we all just decided to ignore it all. But tbh, this has put me off socialising with them all again at home. I am not giving a 14 year old spirits!
To everyone pointing out that kids are allowed to drink in a private residence, yes, I understand that. But medical advice is that children should not be given alcohol regularly (or at all before the age of 15) and certainly not to the point of intoxication as it can affect brain development. I did not allow my two to drink at a young age and I can do without the headache of an argument from them if they watch me giving booze to someone else's kid!