Every parent in the county knows the two or three "main" playgrounds that get mobbed the second the weather turns nice. Fewer know the smaller ones tucked into residential neighborhoods that rarely see more than a handful of families at once.
Glen Rock's smaller municipal parks are a good starting point — less flashy equipment, but plenty of shade and almost never crowded on weekday mornings. A few similar low-key spots exist in neighboring towns too, usually attached to elementary schools and open to the public outside school hours.
If your kid does better with fewer other kids around, these are worth building into your regular rotation instead of defaulting to the big-name parks every time.