The word category of ordinal numbers such as first is somewhat contentious. However, they behaves pretty much exactly like adjectives - which is what they probably are.
Consider the Original Poster's example:
- Reading was very important to John's parents, both of them firsts in their families to go to college.
Here we see first occurring without a following noun. This is similar to what we observe with other adjectives functioning as 'the head of noun phrases', such as the rich or the poor, the disadvantaged or the good, the bad and the ugly.
Adjectives in English do not inflect for number, unlike nouns. In fact this is one of the very reasons why some writers argue that ordinal numbers are adjectives and not nouns: they do not inflect for grammatical number and have no 'plural' forms:
- the elegant description
- *the elegants descriptions
- the first person
- *the firsts people
Even though there is no noun following first in the OP's example, first is still an adjective here. It therefore cannot have an 's' suffix to indicate plurality.
The example should therefore read:
- both of them first in their families to go to college.