Both onion and garlic have very little-to-no nutritional value. Everyone agrees on that
Onion leaves you with an awful odor.
And yes, my personal experience suggests it has a sedative effect. Which kids don't need in the middle of the day
Modern nutritional science backs the poor traditional view of onion, as an item with low nutritional value, and unwanted side-effects (like odor and mild sedation)
E.g. coffee, potato chips
Just because these items make you feel good, doesn't mean they need not be discouraged among young people
Onion / Garlic dont even have these virtues!
If we bring up kids in a culture where they simply cannot do without "chips" or "pickle" or some masala, we are building them up for failure
The upbringing of kids ought to prepare them for that