I
I believe he was never there. Those toilets are on a promenade with cliffs behind and sea in front. You can only walk left or right, and given that he came from the direction of Marske, even if he had turned the wrong way coming out of the toilets, he would have had to turn round to walk back, as he would have been heading out of town the wrong way. There's no way, if his mum was waiting outside, that she could have missed him. Also, with the exception of the sea front, Saltburn is quite hilly, I would imagine given his disabilities, Stephen might have found walking up these hills quite difficult, and surely someone would have noticed him.
The parents are dodgy. They were too happy at finding out they weren't under suspicion. Most people would have cried with relief that it was over. I can't work out which one did it or if both were involved but they definitely know more than they are letting on.