It's American, and a thriller.
Man, wife & little boy go to a theme park for the day.
They separate for a bit, can't remember why, Mum takes little boy in pushchair, then disappears, leaving child unattended in pushchair & father understandably distraught.
I can provide a bit more detail but it's a really excellent book & if someone would like to read it once identified it would probably spoil it for them.