How to refuse offer so as not to offend friend?

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I have known friend for 15 years.
Small village. She is local (& popular) I am neither.
We met as her son (then 7) was part of a small group bullying mine (badly),
She was the only one who behaved properly - taught her son right from wrong & they became best friends. This changed at Secondary School (my son is ASD)
Boys are now both 18. Hers is doing well, Mine is not. Because my son is Autistic things come very hard to him. He was very sad to lose his friend (her son has moved on & stopped acknowledging mine in school /street at all).
Despite all this, friend & I have managed to stay friends (although we have little in common, she is married & my H left, she has a good job, I don't etc)
I have managed not to feel jealous to far as she is kind, works hard & is lovely.

Anyway, I was at her house last week amongst a small group having drinks. I was unusually quiet as I felt I just couldn't 'join in' with any of the conversation about successful kids/ good jobs/ happy marraiges etc. A big a self pity moment! Suddenly she invited me on their family holiday with them for a few days. I was taken aback as it was a genuinely kind offer & felt a bit guilty as I'd been sitting there feeling rather jealous & sorry for myself. We had a coffee a few days later & I explained this. She was nice about it. I have thought it through & I can't take up the offer (I can't leave my Dc & I am not independent enough to manage on hols (it would be a bed whilst they dash about & I am disabled).

How do I phrase a 'no thank you' without sounding rude & churlish?
(perhaps I am?)

'Ive thought about it and unfortunately I'm just not going to be able to leave the kids. That and not being able to be as independent I just don't think it'll work. Thank you so much though, I really appreciate the kind offer.'

Unless I'm missing something it's that simple?

@Coppercup thanks for replying.
Yes, I guess the message is that simple.
I suppose I just feel mixed up about it.
Even my 'best friend' has no clue how impossible it is for me to go away somewhere else for 3/4 days. I think they think I'm fussing over kids.
Reality is, aged 19 & 16, they wouldn't manage without some assistance.
Also, one of their kids would end up on a sofabed so I'd have a room, plus they'll biff around a lot (they are very active & sporty) & I'd be trying to work out the bus into town (I've not been abroad for 20 years & have no confidence) so I think I'd feel a bit of a muppet / spare part, however kind the offer is.

You're overthinking it

Just tell her your having a pity party and dont want to come

Just explain it would be too much for you with everything you have going on, but thank you for the kind offer x

So kind of you to invite me but I wouldn't be able to leave the kids. Hope you have a brilliant time though. Keep it simple x

Is there any need?

I would be warmer:

Hi, I've thought long and hard about the holiday, and I'm sad to say that I've realised it's not feasible for me to come. My children wouldn't manage without me. I would have loved to join, and so appreciate your generous and kindness in inviting me. Thank you! It means a lot. I hope it's a lovely holiday and look forward to hearing all about it.

I am not independent enough to manage on hols (it would be a bed whilst they dash about & I am disabled).
sorry op, do you mean you have care needs yourself/have bed care? Is your friend maybe already aware of this and still happy to invite you?

@DoreenonTill8 sorry if unclear.
they are a hugely fit & sporty family & will do their own thing.
I can do mine too but mine will be very limited as I am ambulant disabled.
I don't think I'll manage public transport from the airport / solo trips into town etc.

@Cas112 well, I was feeling sorry for myself that evening yes.
Now I can just see that I can't leave the country for 3/4 days with no back up for my YP & I will be so exhausted from the trip I'd probably need to sleep mostly.
So I think I'd be stressed about sharing with them as it will look rude/ feeble.

I think I will warmly decline
@SeasickAccountant - that's a good message - thank you.
The thing is, even my lovely friend doesnt really 'get' how difficult it is as my kids mask a lot. But just this week I've had one up at 2am with severe anxiety & one has punched a wall at School & come home with a bleeding hand. It's hot here & they'll forget to drink enough / eat regularly enough. I cant be supporting them as usual from Europe. Even if they do manage much better than I expect (friend thinks they'll be fine) I will be worried about them. Their other parent lives 8miles away & has just been on their 3rd holiday this year but hey ho...
They have little contact & that is DC' choice so can't abandon them to him.
I can truthfully say that even the offer has cheered me up, because it has.