It has to do with creating govt dependency where it is not needed or necessary (by definition, making it universal means that kids who do not need assistance will be getting it), it undermines the principle of parents taking responsibility for providing for the needs of their children, and that weakens families by replacing parents with govt.
The Left has the goal of weakening, ultimately destroying, traditional nuclear families and weakening parental authority of all family structures. We saw this quite clearly during COVID as parents were told they had no right to control what happens to their children at school and that they had no choice in making health decisions for their children in relation to masking and vaccination. But in this school lunch case, broadening govt dependency and substituting parental responsibility with govt, it is another way to undermine and weaken the family.
But, yes, it is sold as "What kind of bad human being opposes feeding hungry kids?' Lol