How much is a blunder? Are they all skinny or just average but don't have the experience?
I'd suggest and much prefer to see body training or workout to build strength and stamina after you mentioned the need to learn to hit vital spots and fight one enemy after another with the least effort possible. Give those adventurers the harshest training possible for the first month, or maybe a full workout at first and in-between for the next months to stay hardened. To train like crazy, and push themselves to the limit every single time. I emphasized the HARD work here because three months is little to nothing. No self-deception and self-enhancement, and just let their body do the talk.
Who are they fighting against, monsters? If so, they also need to learn a lot about characteristics and weak spots to strike, and does that mean the battle evaluation will be the children pitted against seized monsters or from the wilderness or is it just a strength evaluation?
If they fight using melee weapons, then mastering the very basics of it can help them without the need of learning more advanced stuff by building a strong foundation before placing the higher floor. Make the movement faster, the form sturdier, and the strike more accurate. Maybe this applies to magic too?
They may be reluctant, lazy, and can't put in so much effort but the more they do the things they don't want, the more they realize there's nothing more important than putting in the hard work. After all, they are betting their lives on becoming a bit stronger, so they can survive and not die easily.
TLDR; Workout → Master the basic → Set the mindset
Simple over fancy.