I've never been to Punta del Este, but I've been to Viņa del Mar 3x or so. Viņa isn't so much of a "resort" as it is a fairly large city on the beach, with hotels. Few are truly "resort-like." That said, I'd rather be in that sort of setting than a resort setting - having a city to walk through, dining options that don't revolve around resort prices, etc. The ocean water is COLD, and the summer climate is quite similar to central California - foggy/low clouds, cool at night, days can be overcast to quite nice, but generally never too hot. I can't say it is a great place for kids.

If you are looking for a "Conrad" standard of property, Viņa (and most chileno resorts) don't have many of those - Viņa now has a nice Sheraton (I've yet to see it), but most other lodgings in town are quite modest. I've been to "resorts" in other parts of the country, including some of the "nicest" in the particular town (such as Iquique), and they are quite modest, too, but it seems like the people (with their kids) who go there (mostly South Americans) are sufficiently entertained.

You do mean North American winter in the OP, no?