Are the reasons (mom can't stay because of covid-19 and the son is too young to live by himself) legally justifiable reasons?
Unfortunately, not in this case.
The landlord can prevail by arguing that the mom, insofar as a holder of a non-immigrant visa, knew or should have known of the risks of being denied entry in the US. In terms of the Restatement (Second) of Contracts at § 154 (a) & (b), the mom is the "Party [who] Bears the Risk of a Mistake".
Since the risks of which the mom was aware include even arbitrary ones such as the USCIS officer's discretion, an allegation of "supervening circumstances" as per the coronavirus crisis is very unlikely to add any merit to the mom's legal position.
Is the 8 months compensate reasonable?
Although this is admittedly subjective, at first glance it appears reasonable or perhaps even generous (the landlord is proposing the midpoint: half of the remaining 16 months) considering that the landlord is entitled to the completion of the lease.
The "8 months as compensation" is essentially the landlord's proposal to settle his otherwise viable claim of breach of contract. You as lease holders can always attempt to negotiate and make alternative proposals, which is what you are doing already.
Here, the landlord seeks to obviate court proceedings that can be a drag for both him and especially for the mom because she would be unable even to present her arguments in court and/or quasi-judicial proceedings.
A settlement would benefit the landlord also from the standpoint that he would no longer be required to prove mitigation of damages (if that is a requirement in his jurisdiction). And, unless the settlement provides otherwise, the landlord would be free to immediately re-rent the unit without having to reimburse the mom (or the person(s) whose name is in the lease).
That being said, it is important for the lease holder(s) to secure in writing (specifically in the settlement document) a statement from the landlord in the sense that, by virtue of the agreed compensation, the landlord waives any and all claims related to the early termination of the lease. It is always smart to be cautious and preclude the landlord from eventually bringing a claim alleging that the agreed compensation was for something else or that it did not fully settle the controversy(-ies).