It's very likely that, one day, they'll have a conversation about such things. Even if they don't, Peter will probably tell Loki that he can do or be whatever he wants and he's only limited by his own ambition and imagination. So, if Loki decides he wants to be a truck one day, Peter will cheer him on.

"Oh, well - " he blushes a little (more? again?), "sure. And, you're probably not wrong."