I've just watched it with 9yo ds.
It's an excellent WWII spy story. Just please don't call it S&A. Ds loved it-but he loves WWII spy stories (although apparently the plane is a 1950s seaplane. he was thoroughly unimpressed by that)
Spoilers below if you don't want to see them.
The Walkers: John, I actually quite liked, although he is way too old. Only thing is he can't sail. 
and he would definitely not have run from throwing the stone-he'd have owned up straight away (thought exactly the same thing with the cricket ball in The Lion the Witch and the Wardrobe)
Susan was totally wrong. She's nagging at John, quarrelling with him, and not only can't cook, but says she can't too, and impression is she doesn't want to. She's also way too old. And they clearly show they don't like pemmican (which should say bully beef/spam on the tin I think)
Titty/Tatty: name bothered me very little as it was hardly used. I think if I hadn't heard beforehand I wouldn't have noticed. Quite well done, with mostly reasonable characterisation. Age wise about right, but she should be the linchpin of the story and she isn't. A couple of times when she was wrong-sulks when told she can't use matches (in the book only Roger can't) and that was a totally unnecessary change. And when she goes all panicky at the charcoal burners.
Roger: I preferred his character to the book one who always seemed a bit moany and expecting everyone else to pander to him. Well done.
Nancy and Peggy were both too old, and looked like they'd had a 1980s perm. I think it was meant to make them look like sisters, but it didn't. I liked Peggy though. She was the right mixture of trying to stay aloof and wanting to make friends.
Nancy was unpleasant. Not hearty but just sounds unpleasant. If she'd been about 11/12yo I think her attitude could have got away with it. Cross because they'd taken over the island works, but at the age she looks you kind of think "grow up, you know you can't lay claim to it." In the era she was in, and the age she clearly was, she'd have been out working probably!
There were a number of characters from the book that had been made much less pleasant:
Susan and Nancy I've covered. But also Mrs Walker was not a loving mother and sister. She's meant to be fun loving-you can see she'd like to be out with the Amazons so can't properly disapprove of them. Mrs. Jackson has also been made much less pleasant. Totally disapproving of the Walkers and all they do. I think Mr. Jackson's character was taken from Mr Dixon's too (Winter Holiday) The charcoal burners were less pleasant too.
Uncle Jim/Captain Flint was good. I liked the change to him. he was suitably grumpy going to friendly. Incompetent as a spy though-surely he'd have checked he wasn't going to be followed when he was going to send a telegram to his superior and definitely wouldn't have left any risk that the PO lady would leave it on the side for the Russians to see.
Film: First bit was too slow. Several unnecessary scenes. Both items overboard were totally unnecessary and took away from the plot. Roger swimming also seemed a strange addition. Almost as though they thought "oh we have to show we'd followed health and safety" or maybe "oops we need another few minutes on the film, what can we shove in."
Running away from the Amazons was wrong-and they wouldn't have left Roger behind. Susan wouldn't have anyway.
Too many thunder storms too. There should have been glorious sunshine most of the time. They spent more time in the rain than sun I think!
I liked the bit in Rio-another unnecessary bit where Roger goes into the Harbour Master's Office and is told to go away, but I liked the carnival and the business.
The spy bits, if you ignore that they shouldn't be there (!) are good. Not convinced by the preventing the plane from going as I think it would either have tipped the plane up or snapped the rope. I suspect the latter. But I may be wrong.
Could have done with a proper sea battle against Captain Flint and a good scene at the end. The end felt very rushed.
Not a patch on the 1974 one. However if you pretend it's not S&A it's a good story.