*(Correction based on deeper Lal Kitab context: The verse is a sarcastic/reverse warning. Mars in the 1st is actually highly protective of its lineage, but if afflicted, the children suffer.)*
Mars (Mangal) in the 1st house (Lagan) creates a "Manglik" personality—fiery, aggressive, and dominant.
Dr. Rishi Rohit Sharma points out a very specific karmic observation regarding the children of this placement. If Mars is negative or afflicted in the 1st house, the person's aggression and anger often pass down to their children, making the children rebellious or disobedient ("nahi dekhi iski aulaad nek").
However, the hidden remedy is self-control. If the person with Mars in the 1st house learns to control their temper and uses their fiery energy to protect rather than dominate, their children will become their greatest strength.