They are somewhat comparable. The core i3 is a dual core CPU, the A6 is a quad core CPU. However, in terms of processing power a single Intel CPU core is about 30% - 35% more powerful than the AMD core. Most game for kids only use 2 cores at most. And your son will not be able to tell the difference. On the graphics side, both are relatively comparable. In general the Intel HD 4000 is more powerful than the Radeon HD 8400 graphics core, but since it is an ultra low voltage (ULV) Intel CPU, the Intel HD 4000 will be less powerful than to the one in a non-ULV Intel CPU.
Here's an alternative if your son is not going to be carrying the laptop around... a 17.3" HP laptop for $400 with a quad core A8-5550m APU and a more powerful Radeon HD...