He sends a voice message to Thrash: Honesty is important, little one. Your mother will understand it was an accident. After all, trees can be replaced; you cannot. See if Bumblebee can assist with the dent, if not, we'll arrange for someone who can. Oh, and tell Robby congratulations on winning his tournament.
Thrash yelps when he first hears the ping (and subsequently Megatron's voice), considering how on edge he is looking at the young tree he just about felled with his accident. After a listen (and a relisten), he nods, and fiddles with his communication to message him back.
Ah, ha.. uh, thanks, Megatron! I wasn't going to lie to my mom, she knows everything anyways, but— I'll still feel real bad making her feel bad. And it's good to know there's a way to fix this; these dents are nasty! ..if you want to tell Robby, you can let him know yourself; I think he'll like that, and when he's happy, so am I.
He follows up with his brother's phone number— if Optimus has Mo's, he figures Megs can probably get ahold of Robby's, but it's better safe than sorry. At least, now, he's a little less worried about his oopsie.