OPATCONG TOWNSHIP, N.J. — Everyone agrees that this recently rural township, its sleepy streets fringed by old farms, is a fine place to rear children. And in just a few years, hundreds of children have arrived, each like an invoice addressed to taxpayers. Now the town faces another expense, the legal defense of a new ordinance that will, in effect, keep down the number of families moving in. The courts will decide whether the restriction, limiting new multifamily housing units to two bedrooms, crosses a fine line between zoning meant to slow galloping development and zoning meant to keep out...