"you don't apply the law" is a really dishonest way of phrasing this, when "hit them with a small financial penalty for the civil immigration violation and fast-track their green cards" is also an option.

Illegally immigrating to the US is a civil violation, not a criminal one, and far less of a threat to US safety than going 5mph over the speed limit or running red lights. It is entirely lawful for the executive and judicial branches to use discretion and compassion in cases when under-18 US citizens are involved.