EWJP: I agree about modern laxity, but even the older restrictions tell the aged, infirm, pregnant or nursing to consult with one's priest in order to prevent undue harm. Lenten vegan can be the standard for healthy adults, but too-low protein can deplete muscle strength for older people, and too little oil/fat can lead to gallstones. As for eggs and milk, the restrictions are less about what goes in (Matthew 15:11) than about disciplining the body and the human will toward what God wants of us (Matthew 17:19-20). I'll still eat the plainest bread from an old-fashioned bakery even if there's no list of ingredients.
MA: With children, it seems the key would be their age and level of understanding. On the one hand, it's good even for the very young (beginning at around a year or two, depending on what it is that's being limited) to become accustomed to some restrictions in general, even if they're too young to listen to reasons. On the other hand, if it's something they only fight against and perceive that it's nothing more than the parent's imposition, then it's counter to the purpose. Instead, the parents can proactively help children fast/abstain little by little as a way for them to become closer to God. One way could be by teaching children about the Liturgical Year and how Lent matters in the cycle of Redemption.