I would pretty much bet that it is a distinct possibility that these two bat fossils of Onychonycteris could be youngsters who became fossils during there 'crazy teen years'
Given that a bats 'crazy teen years' last a matter of months out of a life span tending to around a decade, I'm not sure it is a likely possibility.
But this is not totally implausible, however the differences in limb/bone ratios are not the only things that make
Onychonycteris distinct from extant and previously know fossilised bat species. So while you might make a case that the limb/bone ratios are explained by immaturity you can't use it to explain the claws. It is also likely that other skeletal cahracteristics such as epiphyseal fusion, bone density and texture or dentition allowed Simmon's et al. to discount the fossils being juveniles.
TTFN,
WK