Can you use 'fun' as an adjective?

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.

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.