Yesterday at the Oscar de la Renta boutique on Madison Avenue, waiters carrying silver trays offered lemonade, white wine and Apple & Eve juice boxes. It was the launch of the designer’s first girls’ dress collection, and New York’s most polished mommies and preschoolers dutifully showed up. The collection, de la Renta’s first for children, does not hold back. Think: elaborate pleats, cap sleeves and ruffles in gorgeous fabrics (like a silk chine taffeta) that a trophy wife would weep over. The prices range from $220 to $330; the designer is donating $100 per dress to the Children’s Defense Fund.