I have a 4 yr old daughter who I recently introduced the feeding process too. She needed a lot of the circle of life talk over the course of about 4 months in order to understand the reason why these furry pets/rats where being killed by the snake. The best way to introduce them IMO is to observe the snake feeding on Frozen thawed first then move into the live protion after they seem fine with the frozen feeding. a lot of reinforcement was necessary in my childs case. She needed to know that rats are the snakes food and that they would die without them. this got me 80% of the way there to live feedings, the other 20% was accomplished through talking and frozen feedings.

Well that's my experience hope your's goes as well as mine did.