Actually to be totally correct, they were walking from a restaurant, and the boy went onto the sand dunes in front of them, and simply DISAPPEARED. They were very close behind him and say it is a mystery how he could have just vanished.
He could have fallen into a hole though which was one thought and covered up again.....
The case is very odd and hard to actually find stuff about it.

Rene Hasee
At one time there were multiple webpages about Rene, Colom, but they seem to have vanished and the information seems to have changed.
Like you, I remember that the parents were walking along the beach with Rene pottering around near them, never far away, investigating everything as kids do, when suddenly he had vanished.
I always felt that he had been tempted from the main beach into the sand dunes by the offer of an ice cream, or some other enducement. Then swiftly over come and taken to the immediately adjacent car park, into a vehivcle and whooshed away.
It was Portugal, at 6 pm on a mid summer evening, in June IIRC. After work. That beach would have had loads of people on it, mainly looking towards the sea ... and if something had happened, other than at the very rear of the beach it would have surely been witnessed by some of them.
The only danger at the rear of the beach was some-one, or some peeps intent on taking him, imo. There is the slightest chance of the sands swallowing him, I suppose, but that is so rare that it seems a no goer to me.
I believe he was tempted into the dunes, grabbed and made off with. However these are only my thoughts.
I would welcome some other likely scenarios bearing in mind the time of day and the likelyhood of a good many people, mainly looking towards the sea, on that beach.
It was reported that the PJ didn't bother to look. They said he had drowned. No witnesses to that.