Generally, children fear the consequences of not doing well in school. One of the basic expectations that you have as parents is that your children do well in school. Although you may scold your child if she performs poorly, you can also pretend to scold your child who performed well. While handing the transcript to your child, look her straight in the eye and say "I am disappointed in your performance this semester." If her "worst" grade was a B+, follow it up, after she starts scanning grades, "because you earned a B+!" Time it so that you surprise her as soon as she looks at the grades.
Before giving your child his grades, tell him that he did something very good and that your family will throw him a party. Decide whether to tell him about his good grades before the party, or whether you want him to find out at the party. Let your child pick the dinner location and have him invite a few of his friends. At some point during dinner, tell him the real reason that you are celebrating him -- his good grades.
Teachers may be willing to prank your child. This works well if, for example, you have a good relationship with the child's teacher. Set up this prank before grades come out. Tell your child that her teacher wants to set up a teacher's conference regarding her current grades. Start the phone or in-person conference seriously, telling your child that you and her teacher wanted to talk about her recent performance. After a few minutes of serious talk, reveal that you all simply wanted to congratulate her on her superior academic performance. Be sure to smile and make your child feel accomplished.
A simple, yet entertaining way to give your child her transcript is to include a reward for earning good grades. Before grades came out, your child may have mentioned a favorite video game, toy or album. Purchase the gift and give it to your child after you both inspect her transcript. Giving it to her after she sees her good performance increases the element of surprise.