It depends on the cassette. Some of them have the larger cogs "permanently" together. In quotes because you can drill out the rivet to separate them. The individual cog cost may not be worth it compared to a new cassette. I think this is an 8 speed, no? If not then likely a 7. I don't think Trek is selling any 6 speeds in their kid bikes, but I could be wrong. In any case, a new cassette might be the simpler route.
Nashbar has some pretty inexpensive ones. You may need to lengthen the chain with the larger cog.