Are there any health concerns with letting children use old LEGO kits?
The most important thing is to make you keep any swallowable magnet out of reach.

This is potentially lethal if two magnets are swallowed and attrach each other from different part of the intestine, as unfortunately did happen to a kid a few years back with some other toy. That's why LEGO has completely redesigned train magnets to be completely encased in the buffers, which are a way too big element to swallow.