Is it safe for a toddler to splash in a fountain in the park?

Yes: Unless the water is obviously dirty or contaminated, there is no danger. Of course, with supervision.

Maybe: Depending on the local use this may or may not be a good idea. Diapered toddlers who may have occasion to sit in the water can also contaminate it with e.coli and a parasite called giardia. Kids who splash that water also put their fingers in their mouth and complete the transfer of water borne germs into their system.A few days later you can have big problems.