That is a wonderful idea, it has been asked for in the past, and I think it may be on the list of future enhancements to consider. But no, there is currently no way (I know of) to get an inactive sim to babysit no matter how closely related or sweet they are unless they are an official teen babysitter (Service Sim, in your home) or are running a professional Day Care center in their own homes. I believe that currently the toddler or baby would be seen by the game as not belonging there and the game itself or the mods would try to transport it home for safety, then one of your active sims would be summoned to return home immediately to take care of it.