Thanks for posting this. It’s certainly a very intriguing premise and has enormous amounts of promise. I very much get the feeling that something exciting is about to happen. As kids love the idea that a magical world is only an opening door away I think they could be drawn in very well.
I do write MG, but 9-11, so not far off. Writing children’s books generally means that the protagonist is a few years older than the age group you are writing for and therefore could be about 10 or 11. I think that first person point of view does draw you in closer but it is of course limited due to having to be at the site of essentially all the action.
My main comment would be to prune out anything that really doesn’t push the story forwards. The chapter describes the day on the beach but very few of the sentences/ideas actually push you towards ending up in the beach hut where presumably the real action starts. I feel that to keep children engaged the dialogue needs to be pacey, preferably funny and anything that’s mundane should be left out, for example the contents of the beach bag/details of cars etc..
It also seems a massive coincidence that they happened to spot the correct beach hut and then they happened to find the key on the beach. I’m sure you are setting this up to be completely magical but my suspension of disbelief is pushed just a little too far for that. If the old man had dropped a note or whispered something like ‘The one with the green door. It will open for you, Polly’ then there could be more reason for the family going ahead to try it. Weird is completely fine but one does have two give a plausible reason for it. Coincidence is not an ideal reason for anything happening and I think you could strengthen up the reasoning for their decisions somewhat.
I also felt there was an awful lot of telling eg at one point it says “she was jubilant”, rather than showing her doing a little dance on the spot or high-fiving her sister.
So I would suggest sticking to story advancing material, funny, quirky, mysterious but leaving out anything mundane if it’s possible to do it without making the flow frankly stilted. I hope that’s helpful.
Good luck with your writing.