The A547 at Meliden, between Rhuddlan and Prestatyn, was closed after 14:00 BST on Saturday after the collision.
The child was taken to hospital, with the extent of his injuries not yet known.
Police confirmed traffic had resumed on the road after 16:00 BST.

British and Irish Lion North, 23, has signed a new contract with Northampton Saints in England's Premiership.
The 23-year-old's previous deal was due to expire next summer and he had been linked with a return to Wales.
"We'd all want to see our best players playing in Wales," Davies said. "Why would we want it any different?"
Davies was Scarlets chief executive when North left them for Saints in 2013, and moved to the Pro Rugby Wales role in September 2014.
He would like to see players such as North back in Wales to offer a "return on investment to the regions that have developed them" as well as to reward fans and to inspire younger players.
Davies added: "From the national squad's point of view, the coaches are very clear that they fundamentally believe that it is a better environment in terms of player management for the players to be playing in Wales.
"They can manage them better in conjunction with the regions."
North, capped 55 times by Wales and also a 2013 Lions tourist in Australia, has appeared 42 times for Saints.
There was speculation he might emulate Wales team-mate and former regional colleague Jonathan Davies, who will return to Scarlets from Clermont Auvergne on a Welsh Rugby Union dual contract next season.
Former Wales centre Tom Shanklin is also disappointed North will remain in England.
"It's probably good for him personally because he's at a very good club and in a very good league," Shanklin told Scrum V Radio.
"But I want to see all the Welsh players back. That's going to strengthen our league and strengthen our product on the field.
"We had some great news Jon Davies is coming back so it's a bit of a shame."

The Dow Jones industrial average slipped 5.23 points to 16,995.13.
The S&P 500 gained 0.31 points to 1,989.57, while the tech-focused Nasdaq index fell 12.22 points to 4,662.16.
US markets mirrored those in Europe after ECB head Mario Draghi said the bank would expand stimulus measures, but did not expect further rate cuts.
Shares in mobile payment company Square fell 6.1%. The company reported on Wednesday that it had seen growth in payment volume over the fourth quarter, but also increasing operating expenses.
Dollar General shares rose 10.7% after the company reported profits of $376m. Its discount competitor Dollar Tree saw its share price rise 4%.
A Denbighshire road has been reopened after a boy was hit by a car, North Wales Police says.
[NEXT_CONCEPT]
Wales wing George North's decision not to return to a Welsh region is a disappointment, says Rugby Wales chief executive Mark Davies.
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(Close): US stocks finished mostly lower following a turbulent trading day and an announcement of new stimulus from the European Central Bank.