In the films there is so much missing, I had friends that don't know the books wonder why major things happen the way they do.
What is missing here is that Hagrid visits Harry on his birthday, which is July 31st. The train leaves on September 1st. So there is the whole month of August between those two scenes. This whole month Harry again lives with the Dursleys. They mysteriously managed to get off the island without the boat, that Hagrid had taken away.
Additionally, in the book it's not Hagrid who takes Harry to King's Cross, instead, Harry manages to persuade Vernon to take him.
"All right, we’ll take you to King’s Cross. We’re going up to London tomorrow anyway, or I wouldn’t bother."
"Why are you going to London?" Harry asked, trying to keep things friendly.
"Taking Dudley to hospital," growled Uncle Vernon. "Got to have that ruddy tail removed before he goes to Smeltings." (PS)
They even enter King’s Cross with Harry, mainly for their own amusement to show Harry that platform nine and three-quarters doesn't exist.
They reached King's Cross at half past ten. Uncle Vernon dumped Harry's trunk onto a cart and wheeled it into the station for him. Harry thought this was strangely kind until Uncle Vernon stopped dead, facing the platforms with a nasty grin on his face.
"Well, there you are, boy. Platform nine -- platform ten. Your platform should be somewhere in the middle, but they don't seem to have built it yet, do they?"
He was quite right, of course. There was a big plastic number nine over one platform and a big plastic number ten over the one next to it, and in the middle, nothing at all.
"Have a good term," said Uncle Vernon with an even nastier smile. He left without another word. Harry turned and saw the Dursleys drive away. All three of them were laughing.(PS)
It is already irresponsible of Hagrid not to tell Harry how to get there, but if in the film Hagrid takes him there and then leaves him alone to find the way, that is even more irresponsible.
Harry not finding the way is one of the key points, because there he meets Ron.