When I rescued two ch youngsters that had NEVER eaten and were at least 2 months old...
I got a nice sized f/t pinkie and used my tongs to gently open the mouth from the side, just a little. I had my husband hold the tongs in place while I got the pinkie just inside the mouth. i then closed the tongs and removed them, leaving the mouse pinkie in its jaws.
I had to do this twice with one bp, the other got it after the first one.
I hope this helps.

Carol