Vet. Now.
I never let Dallas off leash. Not in the green belt, not in the park, not around my condo. Things happen so fast. I scoop him up if a bigger dog runs up to him, and I always hear from other owners "oh ha ha ha my dog is friendly", I reply, "well I am not". It really ticks me off that people just let their big dogs run loose on public land, like it's their own back yard.
I carry a folding knife and if any dog attacked my Dallas, I would have a new dog hide rug in front of my fire place. Others here carry pepper spray and/or walking sticks, all good ideas.
I hope you file a police report or a report with animal control. And send the pit's owners the bills. If they don't pay take them to small claims court. Next time this dog may want to take a chunk out of someone's kid.
Best to you and the little one, let us now how you both are doing.