[Mal simply looks thrilled. It's not until he looks back and sees Bart holding on for dear life and looking terrified that he slows his horse down. As he slows, he leans over and reaches for the reins of Barth's horse as well.]
Woah girl!
[His voice is calm as he gets the horses to a more manageable pace.]
Sorry kid, you shoulda told me you weren't so into goin' that fast.