How can human infants express chymosin with only a pseudogene at their disposal?
TL;DR: Chymosin is similar to pepsin and I couldn't find any evidence of functional/expressed chymosin gene in human genome.

It seems like a common misconception that chymosin is functional in humans. Already in 1940s it has been shown that rennin (aka chymosin) is absent from "gastric juice" in adult humans.

Genetically there is only pseudo-gene for chymosin, as wiki and other sources state. At the same link that leads to chymosin gene page you can find some literature on cloning and structural analysis of chymosin pseudo-gene.

If you use BLAST for finding homologous proteins to chymosin from, say Bos  mutus (aka yak), you can find that in humans closes sequence can be attributed to pepsinogen and similar proteins. Which very much might mean that what is called chymosin in other animals is pepsin in humans, or there is some convergence/divergence which eliminated chymosin in humans. But this is only a speculation.

You can try to scan Google scholar for more sources.

NB link to BLAST search will expire in ~24 hrs.