Thornborough works for the Goodbook Company but his other job is to run the children's group at his church in Wimbledon each week.
In his view, church children's workers are a "bit undervalued", sometimes by themselves, sometimes by the parents, and sometimes by the church leadership may just think they are there to "take the shreakers and the wailers out of the church building".
With research indicating that most Christians embrace the faith before the age of 17, Thornborough said the work being done in children's ministry was of "crucial importance".
"Evangelists who go out hunting for people do a great job but the biggest work of evangelism is what you do in your children's group," he said.
"You are the frontline evangelists doing the groundwork so that there will be a church in the next generation."
Instead of just seeing their task as taking care of the church's children for an hour or two each week, Thornborough said the job of everyone in children's ministry was to grow young people to maturity.
"You are not a childminder, you are in Christian ministry. Jesus has given you the Holy Spirit and the Word of God so that you can share the Good News of Jesus Christ with children."
He continued: "I'm not minding the children. I'm using my God-given wisdom so that these children will grow into mature disciples."
Thornborough was speaking to Christians involved in children's and youth ministry at the Jesus Then And Now conference in central London on Saturday.
He made an appeal for more men to get involved in children's ministry.
"We don't want boys growing up in Sunday school believing that Christianity is for girls only because they only saw female leaders in Sunday school," he said.
Andrew Page, missionary and creator of the Mark Drama, encouraged children's ministry workers to first be excited about Jesus and his life before trying to teach him to others.
"If we have a new excitement about Jesus then and now, then we will have a new motivation to talk about Jesus to children and young people," he said.
Page encouraged children's workers not to compare themselves to each other and recognise that their shortcomings were not a bar to working in children's ministry.
"What changes the lives of children in our groups is not us but the Word of God," he said.
"You see others and think they're so gifted and only you are a mess. We're all a bit of a mess, Jesus specialises in using people who are a bit of a mess.
"He's not calling you to be someone else. He's calling you to be you."
He added: "You are almost certainly not the most gifted person in your church but be servant-minded and children will see that in your example."