The first sentence says that he spent 30 minutes playing in the yard, and then the bug bit him. It uses the pluperfect tense, the play happened before the bite, but both happened in the past. The second says simply that at the moment when the bug bit him, he was in the process of playing in the yard.

In the first sentence you are talking about an action that had been in progress before another one in the past. Let me show you another example: Two years ago, I moved to Curitiba. I had been living in Sao Paulo before (You were living there many times before). In the second you are talking about an action that was still going on when something happened. "They were waiting in the line when they saw each other for the first time" Now, I'll show you a sentences which you'll see the difference between them: "While they were talking, they found out they had been living in the same neighborhood (uk. neighbourhood) for 10 years." (Enquanto eles estavam falando, eles perceberam que tinhan estado morando dez anos na mesma vizinhança). I hope I've helped to you. See you!

Good question, Giovana! I have the same doubt! =(