I don't think it was a random abduction, Ollie was with his father who was holding his hand in the bar, abductor would need to be very close waiting for the moment when Tony lets go of the hand. Very risky too as Tony could turn, notice any moment. Also abductor would have to make Ollie walk with him. Ollie was what, about 6 yrs old? even if the screaming would be muted by the shouting crowd, Ollie would still probably resist, try to push away. Abductor would take a big risk going into the crowd.
However if Ollie knew his abductor, he possibly lets go of Tony's hand to move closer to abductor, abductor could easily lure him out of the crowd and walk away without problem. And if Tony notices anything, the abductor can make up a story as coming to meet them both etc.
That would make Emily a suspect or perhaps Sylvie- hotel owner as she was fairly friendly with Ollie and would have known that they went swimming (ollie went back to the hotel to get his trunks). It makes me also think that the pedophile ring could be this town's dirty secret - a few peoples involved and some covering up for the family members.