I believe you have very serious cause for concern, for yourself, for the baby, for other persons and animals in the area, and for the mother of the baby.
You may wish to video the situation with the dog while you are getting water. Burn a dvd, mail and/or email the park owner. Keep a copy. Talk to the manager and write a letter to the park owner or office or corporation.
I do not know how much information you have on pit bulls and their dangers and history of attacks, but here is a start:
https://www.nationalpitbullvictimawareness.org/
http://www.animals24-7.org/2017/08/08/pit-stop-archive/
and you may wish to look at youtube: “pit bulls jumping fences”, plus much information has been posted here on FR, keyword “pit bull”.
Woman cited for firing gun to scare off pit bulls that killed pet (Artesia, NM)