Krish Kandiah, founder of Christian charity Home for Good and president of the London School of Theology, was speaking after 292 MPs - mostly Conservatives - voted against the measure, versus 276 in favour.
A campaign involving Save the Children and other Christian and secular leaders and charities have been calling for Britain to not only help refugees still in Syria, but also those who've made the potentially fatal journey to Europe.
The govermnent has already pledged to take in 20,000 Syrian refugees by 2020, and recently pledged to take another 3000 unaccompanied children from the Middle East and North Africa region.
It has also said that it does not want to implement measures which could encourage migrants and refugees to make a potentially perilous journey with people traffickers in the hope of gaining asylum once in Europe.
Krish Kandiah told Premier: "We're really disappointed that it was voted down.
"Children caught in these awful situations, incredibly vulnerable to child exploitation, sexual trafficking, kidnapping and abuse, they still need us.
"It's not over, we're encouraging people to continue writing to their MPs.
"We definitely need to be preventing the causes of the migration that are horrific, and causing people to make unimaginably difficult decisions.
"But on the other hand, we can't abandon children that are here on our doorstep."
Immigration Minister James Brokenshire said: "The UK government is committed to providing life-saving support and assistance to the vulnerable children who have been unjustly impacted by this ongoing humanitarian crisis.
"We have always been clear that the vast majority of vulnerable children are better off remaining in host countries in the region so they can be reunited with surviving family members.
"However, there are exceptional circumstances in which it is in a child's best interests to be resettled in the UK.
"We have engaged with the UNHCR and a number of NGOs on the best way to provide protection to refugee children and ensure their welfare and safety remain at the heart of every decision made."
Listen to Premier's Aaron James speaking to Krish Kandiah:
