@Crimble0 I'm not a fan of the "infants stay free" policy, and this is a great example of why Airbnb really should kill it. But it is what it is. In the booking process, it's really obvious and explicit that "infant" is under 2 and "child" is 2-12. You can't possibly miss it.
It would be totally reasonable to ask the guests to revise their request and pay the appropriate fee. If they aren't agreeable to that, they will have outed themselves as cheapskates who would have been terrible guests anyway, so declining them would be a bullet dodged.