What is the actual risk when my 6-month old chews on a USB battery pack?
If the moisture makes its way inside the battery casing, could it cause any sort of chemical leakage? If the ports get wet, could it short the battery and start a fire? Am I being paranoid or irresponsible here?


Yes and yes and irresponsible. 

There are a multitude of special laws for designing toys and items for children. A non - waterproof high capacity lithium cell for a baby is incredibly bad. Hot glue is likely not kid safe either. Cables are choke hazards.