[2019] My 6-year-old got banned for XP farming because he bought the Jurassic Park Jeep...?

Hi guys, hoping someone can help me out here.

Let me start off by giving a bit of background: I love playing Rocket League on my PC. My kids like playing split-screen with me. When crossplay capabilities came out, I got the game on Xbox for them. My 6-year-old and 4-year-old play split screen on the Xbox on a dedicated account; I play with them on the PC. I never play on their account on the Xbox; they are the only ones who use that account.

Today they asked if they could use their allowances to each buy a new car. One chose the Fast and Furious Nissan; the other chose the Jurassic Park Jeep. As soon as the purchases went through, a message popped up saying that the account had been banned from online play for 48 hours due to XP farming. Now, they do play online sometimes (usually in Rumble mode because they like the "superpowers." But they definitely haven't done anything to "XP farm."

Any idea what's happened or who I can go to so that we can get this sorted out so that I can play the game online with my kids again?