Pupils from three schools took part in a road safety exercise with Dyfed-Powys Police.
They used hand-held devices to check speeds in 20mph zones - and anyone going above the limit was told to pull over by officers.
The children then approached the offending drivers and warned them about the dangers of driving too fast.
Police community support officer Josh Jones said: "I think that the devastating consequences of speeding are made more powerful when they are spoken by a child.
"It had an impact on those who were pulled over and some were quite shocked as well as embarrassed.
"The idea is that it will make people stick to the speed limit - especially when near schools."
Fourteen drivers in Llanelli, Carmarthenshire, were warned for speeding on Wednesday - by children.