What resort are you staying at?
The ride from MoBay (Sangster Airpor MBJt) to Negril is safe, do you have a shuttle lined up or is the resort including it?
When traveling from your resort to areas around town, there will most likely be a Tour Operator working out of the resort to help you
There really isn't any decent fishing in and around Negril, there is a charter boat at the Pier but I would find other things to do
Pelican Bar is a long way from Negril, it is down near Treasure Beach/Black River area on the south shore in St Elizabeth Parrish
The beach and the pool will be what keeps the kids entertained, Lots of sunscreen and keep them hydrated, the Aug. Sun can be intense (Take a can of gatorade in your luggage, just add water and the kids love it)
Take the kids to see the cliff divers @ Ricks & down to the water park(I can't believe I said that), the caves at Xtabi, out to get some jerk chicken from a street vendor and find some early music on the beach. Have Fun