To his credit, Jack does his best not too look too smug, though he doesn't make any attempt not to openly enjoy how Ianto looks.
"I think we should head up to the top of the tallest slide to make the most of crashing into the chill of the water!" he grins, catching Ianto's hand again to lead him outside. They don't hold hands an awful lot in the normal way of things, but this is very much like a date and hand-holding somehow feels right.
Or perhaps it's a way of publicly showing his claim on Ianto, along with the renewed hickey showing up dark on Ianto's neck.