The problems associated with this credit system were famously analyzed after the Capitol Hill Babysitting Co-op ran into, and resolved them.
https://en.wikipedia.org/wiki/Capitol_Hill_Babysitting_Co-op
https://en.wikipedia.org/wiki/Capitol_Hill_Babysitting_Co-op
Plus, at a measly 20 families right now, all in DC, it's unclear whether this model would work for bigger or more heterogenous groups. Probably at a smaller scale, the expectation of reciprocity alone would be enough; at larger scales, "financiers" would probably try to game the system and skew exchange rates. In my (limited) experience, this sort of thing only works when a group is both small enough and homogenous enough that the primary driver is community and mutual respect, not any real sort of economic game theory. At that scale members don't tend to hyper-optimize for individual "wealth" over collective gain (yet).