Question: I was walking to school (England) with my eight-year-old talking about stuff she'd been doing the day before. At some point she said that something had been "...funner..." than something else.

I did not immediately realise that 'fun' is, historically at least, a noun and NOT an adjective. So I was wondering in my head why 'fun' (as an adjective) sounded wrong when made into the standard comparative for a single syllable adjective (i.e. 'funner').

I have since looked 'fun' up and realise that 'officially' 'fun' is not an adjective (see OED, and <https://grammarianism.wordpress.com/2015/06/05/can-fun-be-an-adjective/>).

I'm wondering whether adjectival usage is increasing. Whether it is regional (I'm guessing it's an American import); And whether, adjectivally, it is commoner to say *funner* and *funnest*, or *more fun* and *most fun*. The latter sound better to me, presumably because they are usages that are found when fun is used as a noun.

Answer: 
Nowadays it's common to use "fun" as an adjective. 

Fun is used for talking about something that is enjoyable
or someone that you enjoy being with. 

1) Barcelona is a fun city.
2) Our day at the beach was really fun.

The examples are collected from Macmillan dictionary. And by saying " more /most fun ", you will be in the safe side.

<http://www.quickanddirtytips.com/education/grammar/who-says-fun-cant-be-an-adjective> : This article is worth a read.
No, definitely not!
I might say it is the 'most fun city', but it is not really my (British)style.
I would be more likely to say "I had more fun in Barcelona than in any other city"
I can spare the extra time and ink for a more 'elegant' sentence!