One tip for success (though it kinda only works at its best with ABS set to '1'): start braking when you get roughly halfway between the 100m and 50m marker boards.
Rule isn't 100% applicable to every track in the RB Jnr race - you can get away with the big, looping right hander at Rome and the 90 degrees left turn at Brands Hatch by braking at or just before the final board - but it should mean you'll fly past the AI drivers who (based on personal experience and observation) slam on the anchors way earlier.
Sometimes, the NPCs even brake as they're going through the corner, which (as long as you're going around the outside, as @
FussyFez stated) should make overtaking them at least a fair bit easier.
Quick Q, though (and should've probably asked this before devoting three paragraphs to what is basically a 'start braking when you reach this spot' tip...): what are you particularly struggling with in the RB Jnr series?