If a person hears a Beracha (blessing) from a child, does he answer amen (so be it) to that Beracha? If the child has reached what we call higia lechinuch, the child has reached an age about six or seven years old, understands how to formulate a Beracha, what a Beracha means, and what it means to bless Hashem, and is actually picking up a fruit or a food and making the proper Beracha, then it is appropriate for a person to answer amen to that Beracha. It is considered a valid Beracha.
However, if the child is under the age of lechinuch and doesn't really know how to make a Beracha, and you're kind of just saying the words back and forth, then even if the child is about to eat the food and is making the Beracha because you're sort of spoon-feeding them that Beracha, no amen is said. Certainly, if you're just teaching children in the classroom what the proper Beracha is and they're reciting the Beracha, no amen is said, because it doesn't constitute a valid Beracha. A person does not say amen unless the Beracha is considered valid or appropriate for that child to make.