Yes, we are permitted to say Amen to a child's blessing when he is reciting a real blessing (such as right before he eats an apple). If he is only learning the blessing, though (for example, he is sitting with his father or rebbi who is teaching him to say "Baruch" and then "Atah" and then "Hash-m" and so on), one certainly does not answer Amen after his blessing, because it really was not a blessing, but said in practice -- the same way we do not say Amen to an adult's blessing said while practicing.

All the best,

Y. Shaw