"I am not an engineer, so please forgive the lack of clarity in my explanation. The Avidyne engine is an impulse propulsion system developed in the twenty-third century for interstellar spacecraft. Starfleet, last I heard, is currently looking for a replacement to the Avidynes, however. Most humanoids find them to be--" What had Scotty called it? Ah, yes. "--touchy."
Spock crossed his arms over his chest, as though that helped him to concentrate on his explanation of starship mechanics. He was more familiar with computer systems than the actual construction of the devices, but like all Starfleet personnel, he had taken the course on Starship Propulsion Systems.
"Nacelles are the outboard engine housing structures on the spacecraft. In warp-class vessels, the nacelles house the warp coils which create a subspace displacement field that allows the vessel to travel faster than light. The warp coils are fed directly through plasma conduits from the warp core assembly. The nacelles are separated from the rest of the ship to prevent radiation leaks."
He paused, feeling that would be sufficient for a young girl. However, he could delve into more detail if she wished. But, like he said, warp core assembly was not his specialty.