If they are riding a phazer I wouldn't worry about a throttle block.
It doesn't have that much power to begin with and they may need that power for turning or climbing, or to just pin it for short blasts.
Just take em out and take it easy. They will get used to it pretty fast. A phazer is a good sled to start on.
ya i started out on a 340 and then on to a phazer,,,i agree they need all of the throttle tho