Hello! When you cross your fingers, it means that you want to be lucky and you hope for the best even if it's difficult to. The boy wants a bench press, but it will be difficult for him to get one so he better hope for the best and expect to be lucky if he wants to have it. I hope to have helped you!

As Dee said, The cross your fingers means to wish for good luck :)

It is to hope for the best or hope that things work out in their favour, so in this context, the person is being optimistic and hopeful that they will get a bench press as a Christmas present. It can also be used as an idiom. For example: I have studied for that exam for 2 days, hopefully, I get a good grade for it. Fingers crossed!

Dee is correct