When we first moved to Union City my son was four and my mission was to find a good playground. Many of the neighborhood playgrounds were small and secluded and so we ended up at Kennedy Park.
The play structures are interesting and kept my son entertained. The area is very open and visible and therefore felt safe. But it is right next to one of the busiest streets in Union City and right near the BART station so it's not quiet. However, the play area is inset from the street far enough that I didn't worry about my son darting into the street.
Kennedy Park is in the heart of Union City. It is an extremely busy park - which can be good or bad depending on what you're looking for. On one side is Decoto Rd with cars racing by and on the other side is the BART station with trains zooming overhead every few minutes. My 3.5 year old, who is a huge Thomas fan, is thrilled that "Thomas" zooms by every so often. This park can be noisy, it can be loud and pretty crowded at times because the park is so central.
This is the largest community park in Union City and one of the oldest but everything has been completely remodeled and updated to meet safety standards. The park offers two play structures - one for older kids and one for toddlers.